Form DEFM14A Nightstar Therapeutics

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant  ☒                            Filed by a Party other than the Registrant  ☐

Check the appropriate box:

NIGHTSTAR THERAPEUTICS PLC

(Name of Registrant as Specified in its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

THIS DOCUMENT IS IMPORTANT AND REQUIRES YOUR IMMEDIATE ATTENTION. THIS DOCUMENT CONTAINS A PROPOSAL WHICH, IF IMPLEMENTED, WILL RESULT IN THE CANCELLATION OF THE LISTING OF NIGHTSTAR THERAPEUTICS PLC’S ADSs ON THE NASDAQ GLOBAL SELECT MARKET. PART II (EXPLANATORY STATEMENT) OF THIS DOCUMENT COMPRISES AN EXPLANATORY STATEMENT IN COMPLIANCE WITH SECTION 897 OF THE COMPANIES ACT 2006.

If you are in any doubt as to the action you should take, you should consult your stockbroker, bank manager, attorney, solicitor, accountant or other independent professional adviser who, if you are taking advice in the United Kingdom, is authorised pursuant to the Financial Services and Markets Act 2000 or, if you are in a territory outside the United Kingdom, is an appropriately authorised independent financial adviser.

If you sell, or have sold or otherwise transferred all of your Nightstar Shares or Nightstar ADSs, please send this document (but not any accompanying personalised documents) at once to the purchaser or transferee, or to the bank, stockbroker or other agent through whom the sale or transfer was effected for delivery to the purchaser or transferee. If you have sold or otherwise transferred part of your holding of Nightstar Shares or Nightstar ADSs, please consult the bank, stockbroker or other agent through whom the sale or transfer was effected.

The distribution of this document in jurisdictions other than the United Kingdom and United States may be restricted by the laws of those jurisdictions, and therefore persons into whose possession this document and any accompanying documents come should inform themselves about, and observe, any such restrictions. Any failure to comply with those restrictions may constitute a violation of the securities laws of any such jurisdiction.

The accompanying Forms of Proxy and ADS Voting Instruction Card are personalised. If you have recently purchased or been transferred Nightstar Shares or Nightstar ADSs, you should contact Nightstar’s registrars, Computershare, on the telephone number set out on page xvii of this document to obtain replacements for these documents.

Recommended acquisition of

NIGHTSTAR THERAPEUTICS PLC

by

TUNGSTEN BIDCO LIMITED

A WHOLLY-OWNED SUBSIDIARY OF

BIOGEN SWITZERLAND HOLDINGS GMBH

to be effected by means of a

scheme of arrangement under Part 26 of

the Companies Act 2006

Nightstar Shareholders and Nightstar ADS Holders should read carefully the whole of this document and the accompanying Forms of Proxy (or, if applicable, the ADS Voting Instruction Card). Your attention is drawn to the letter from the Chairman of Nightstar in Part I (Letter from the Chairman of Nightstar Therapeutics plc) of this document, which contains the unanimous recommendation of the Nightstar Directors that you vote in favour of the Scheme at the Court Meeting and in favour of the Resolutions to be proposed at the General Meeting. An explanatory statement explaining the Scheme in greater detail is set out in Part II (Explanatory Statement) of this document.

Notices of the Court Meeting and the General Meeting, both of which are to be held at the offices of Nightstar’s solicitors, Skadden, at 40 Bank Street, London E14 5DS on 8 May 2019, are set out at the beginning of this document. The General Meeting will start at 10.30 a.m. and the Court Meeting will start at 10.40 a.m. (or as soon thereafter as the General Meeting has been concluded or adjourned).

The action to be taken by Nightstar Shareholders in respect of the Court Meeting and General Meeting is set out on pages xviii and xix. Whether or not you intend to be present at the Court Meeting and/or the General Meeting, please complete and sign both Forms of Proxy accompanying this document, BLUE for the Court Meeting and WHITE for the General Meeting, in accordance with the instructions set out on pages vi to viii of this document (Notice of Court Meeting) and pages ix to xiii of this document (Notice of General Meeting) of this document and return them using the prepaid envelope provided to Nightstar’s registrars, Computershare, as soon as possible, and in any event so as to be received by Computershare at The Pavilions, Bridgwater Road, Bristol, BS13 8AE, United Kingdom not later than 10.30 a.m. on 3 May 2019 in the case of the General Meeting and not later than 10.40 a.m. on 3 May 2019 in the case of the Court Meeting or, in the case of any adjournment, not later than 48 hours (excluding any part of a day that is not a Business Day) before the time fixed for the holding of the adjourned meeting. Alternatively, the Form of Proxy for the Court Meeting (but not the General Meeting) may be handed to the Chairman of the Court Meeting at the commencement of that meeting. Forms of Proxy returned by fax will not be accepted. You can also lodge your proxy vote online at www.eproxyappointment.com, so as to be received not later than 48 hours before the relevant meeting. The return of a completed Form of Proxy, the electronic appointment of a proxy or the submission of a proxy via CREST will not prevent you from attending the Court Meeting and/or the General Meeting, or any adjournment thereof, and voting in person if you so wish and if you are entitled to do so.

Registered Nightstar ADS Holders are asked to complete and return their ADS Voting Instruction Cards in accordance with the instructions on the ADS Voting Instruction Card as soon as possible. If you hold such Nightstar ADSs indirectly, you must rely on the procedures of the broker, bank or other nominee through which you hold your Nightstar ADSs.

This document is dated 9 April 2019 and is first being mailed to Nightstar Shareholders on or about 9 April 2019.

If you hold your Nightstar Shares in uncertificated form through CREST, you may vote using the CREST Proxy Voting Service in accordance with the procedures set out in the CREST Manual (please also refer to the accompanying notes to the notice of the General Meeting set out at the end of this document). Proxies submitted via CREST (under CREST participant ID 3RA50) must be received by Computershare not later than 10.30 a.m. on 3 May 2019 in the case of the General Meeting and not later than 10.40 a.m. on 3 May 2019 in the case of the Court Meeting or, in the case of any adjournment, not later than 48 hours (excluding any part of a day that is not a Business Day) before the time fixed for the holding of the adjourned meeting.

Capitalised words and phrases used in this document shall have the meanings given to them in Part VII (Definitions).

Centerview UK, which is authorised and regulated by the FCA, and Centerview are acting exclusively for Nightstar and no one else in connection with the matters referred to in this document. Centerview UK and Centerview are not and will not be responsible to anyone other than Nightstar for providing the protections afforded to its clients or for providing advice in connection with the contents of this document or any matter referred to in this document.

Goldman Sachs International, which is authorised by the Prudential Regulation Authority and regulated by the Financial Conduct Authority and the Prudential Regulation Authority in the United Kingdom, is acting exclusively for Bidder and no one else in connection with the matters referred to in this document and will not be responsible to anyone other than Bidder for providing the protections afforded to clients of Goldman Sachs International or for providing advice in connection with the matters referred to in this document.

IMPORTANT NOTICE

This document has been prepared for the purposes of complying with English law and U.S. securities law and the information disclosed herein may not be the same as that which would have been disclosed if this document had been prepared in accordance with the laws of any other jurisdiction.

The distribution of this document in jurisdictions other than the United Kingdom and the United States may be restricted by law, and therefore, persons into whose possession this document comes should inform themselves about, and should observe, such restrictions. Any failure to comply with the restrictions may constitute a violation of the securities laws of any such jurisdiction. This document does not constitute an offer or an invitation to purchase or subscribe for any securities, or a solicitation of an offer to buy any securities, pursuant to the document or otherwise, in any jurisdiction in which such offer or solicitation is unlawful.

The statements contained herein are made as at the date of this document, unless some other time is specified in relation to them, and service of this document shall not give rise to any implication that there has been no change in the facts set forth herein since such date. Nothing contained in this document shall be deemed to be a forecast and service of this document shall not give rise to any implication that there has been no change in the facts set out in this document since such date. Nothing contained in this document shall be deemed to be a forecast, projection or estimate of the future financial performance of Nightstar or Bidder except where otherwise expressly stated. Neither Nightstar nor Bidco nor Bidder intends, or undertakes any obligation, to update information contained in this document, except as required by applicable law.

No person has been authorised to make any representations on behalf of Nightstar, the Nightstar Group, Bidco, Bidder or the Bidder Group concerning the Acquisition or the Scheme which are inconsistent with the statements contained in this document and any such representations, if made, may not be relied upon as having been authorised.

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WARNING REGARDING FORWARD-LOOKING STATEMENTS

All statements included in this document, other than statements or characterisations of historical fact, are forward-looking statements within the meaning of the federal securities laws, including Section 21E of the U.S. Exchange Act, and other securities laws. Whenever Nightstar uses words such as “believe,” “expect,” “anticipate,” “intend,” “plan,” “estimate,” “will,” “may,” “predict,” “could,” “seek,” “forecast” and negatives or derivatives of these or similar expressions, they are making forward-looking statements. Examples of such forward-looking statements include, but are not limited to, references to the anticipated benefits of the Acquisition and the expected time of effectiveness of the Acquisition. These forward-looking statements are based upon Nightstar’s present intent, beliefs or expectations, but forward-looking statements are not guaranteed to occur and may not occur.

Nightstar Shareholders are cautioned that any forward-looking statements are not guarantees of future performance and may involve significant risks and uncertainties, and that actual results may vary materially from those in the forward-looking statements. Important risk factors that may cause Nightstar’s actual results to differ materially from their forward-looking statements include, but are not limited to: (1) the Acquisition is subject to the satisfaction or waiver of certain conditions, including the receipt of requisite approvals by Nightstar Shareholders, the sanction of the Scheme by the Court, and the expiration or termination of any applicable waiting periods under the U.S. HSR Act, which conditions may not be satisfied or waived; (2) uncertainties as to the timing of the consummation of the Acquisition and the ability of each party to consummate the Acquisition; (3) the risk that the Acquisition disrupts the parties’ current operations or affects their ability to retain or recruit key employees; (4) the possible diversion of management time on Acquisition-related issues; (5) litigation relating to the Acquisition; (6) unexpected costs, charges or expenses resulting from the Acquisition; and (7) potential adverse reactions or changes to business relationships resulting from the announcement or completion of the Acquisition.

The information contained in Nightstar’s filings with the SEC, including in Nightstar’s Form 10-K for the year ended 31 December 2018, identifies other important factors that could cause actual results to differ materially from those stated in or implied by the forward-looking statements in this document. Nightstar’s filings with the SEC are available on the SEC’s website at www.sec.gov. You should not place undue reliance upon forward-looking statements. Except as required by law, Nightstar does not intend to update or change any forward-looking statements as a result of new information, future events or otherwise.

INFORMATION FOR OVERSEAS SHAREHOLDERS

The release, publication or distribution of this document in jurisdictions other than the United Kingdom and the United States may be restricted by law and therefore any persons who are subject to the laws of any jurisdiction other than the United Kingdom or the United States (including Restricted Jurisdictions) should inform themselves about, and observe, any applicable legal or regulatory requirements. In particular, the ability of persons who are not resident in the United Kingdom or the United States or who are subject to the laws of another jurisdiction to vote their Nightstar Scheme Shares in respect of the Scheme at the Court Meeting, or to execute and deliver Forms of Proxy appointing another to vote at the Court Meeting on their behalf, may be affected by the laws of the relevant jurisdictions in which they are located or to which they are subject. Any failure to comply with applicable legal or regulatory requirements of any jurisdiction may constitute a violation of securities laws in that jurisdiction. This document has been prepared for the purpose of complying with English law and U.S. proxy rules and the information disclosed may not be the same as that which would have been disclosed if this document had been prepared in accordance with the laws of jurisdictions outside the United Kingdom and the United States.

Copies of this document and any formal documentation relating to the Acquisition are not being, and must not be, directly or indirectly, mailed or otherwise forwarded, distributed or sent in or into or from any Restricted

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Jurisdiction or any jurisdiction where to do so would constitute a violation of the laws of such jurisdiction and persons receiving such documents (including custodians, nominees and trustees) must not mail or otherwise forward, distribute or send them in or into or from any Restricted Jurisdiction. Doing so may render invalid any related purported vote in respect of the Acquisition.

If the Acquisition is implemented by way of an Offer, the Offer may not (unless otherwise permitted by applicable law and regulation) be made, directly or indirectly, in or into or by use of the mails or any other means or instrumentality (including, without limitation, facsimile, e-mail or other electronic transmission, telex or telephone) of interstate or foreign commerce of, or any facility of a national, state or other securities exchange of any Restricted Jurisdiction and the Acquisition will not be capable of acceptance by any such use, means, instrumentality or facilities or from within any Restricted Jurisdiction.

NOTICE TO UNITED STATES NIGHTSTAR SHAREHOLDERS

The Acquisition relates to the shares of a U.K. company and is being made by means of a scheme of arrangement provided for under Part 26 of the Companies Act 2006. The Acquisition is to be implemented by way of a scheme of arrangement. If, in the future, Bidder or Bidco exercises its right to implement the Acquisition by way of an Offer, subject to the terms of the Implementation Agreement, and determines to extend the Offer into the U.S., the Acquisition will be made in compliance with applicable U.S. laws and regulations.

It may be difficult for U.S. Nightstar Shareholders and Nightstar ADS Holders to enforce their rights and any claim arising out of the U.S. federal securities laws, because Nightstar is located outside of the U.S., and some or all of its officers and directors are residents of countries outside of the U.S. U.S. Nightstar Shareholders and Nightstar ADS Holders may not be able to sue a non-U.S. company or its officers or directors in a non-U.S. court for violations of the U.S. securities laws. Further, it may be difficult to compel a non-U.S. company and its affiliates to subject themselves to a U.S. court’s judgment.

U.S. Nightstar Shareholders and Nightstar ADS Holders also should be aware that the transaction contemplated herein may have tax consequences in the U.S. A summary of certain United Kingdom and United States taxation consequences of the implementation of the Scheme for certain Nightstar Shareholders and Nightstar ADS Holders is set out in paragraph 6 of Part V (Additional Information) of this document. U.S. Nightstar Shareholders and Nightstar ADS Holders are urged to consult with legal, tax and financial advisers in connection with making a decision regarding this transaction.

PUBLICATION ON WEBSITE

A copy of this document, together with all information incorporated into this document by reference to another source, will be made available, subject to certain restrictions relating to persons resident in, or subject to the laws and/or regulations, of any Restricted Jurisdiction or resident in any jurisdiction where the extension or availability of the Acquisition would breach any applicable law, on Nightstar’s website, at https://ir.nightstartx.com, on the date following publication of this document. For the avoidance of doubt, neither the contents of such website nor the contents of any website accessible from hyperlinks on such website (or any other websites referred to in this document) are incorporated into, or form part of, this document.

ROUNDING

Certain figures included in this document have been subjected to rounding adjustments. Accordingly, figures shown for the same category presented in different tables may vary slightly and figures shown as totals in certain tables may not be an arithmetical aggregation of the figures that precede them.

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It is important that, for the Court Meeting, as many votes as possible are cast so that the Court may be satisfied that there is a fair representation of the opinion of Nightstar Scheme Shareholders. Therefore, whether or not you plan to attend the Nightstar Shareholder Meetings, please complete and sign both the enclosed BLUE and WHITE Forms of Proxy, or deliver your voting instructions by one of the other methods mentioned below, as soon as possible.

Date

This document is published on 9 April 2019.

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NOTICE OF COURT MEETING

IN THE MATTER OF NIGHTSTAR THERAPEUTICS PLC

and

IN THE MATTER OF THE COMPANIES ACT 2006

NOTICE IS HEREBY GIVEN that by an Order dated 5 April 2019 made in the above matters, the Court has given permission for a meeting (the “Court Meeting”) to be convened of the holders of Nightstar Scheme Shares (as defined in the Scheme of Arrangement hereinafter mentioned) for the purpose of considering and, if thought fit, approving (with or subject to any modification, addition or condition which the Company and Biogen Switzerland Holdings GmbH may agree and which the Court approves) a scheme of arrangement (the “Scheme of Arrangement”) proposed to be made between (i) Nightstar Therapeutics plc, a public limited company incorporated in England and Wales with its registered office at 10 Midford Place, London, W1T 5BJ (the “Company” or “Nightstar”); and (ii) the holders of Nightstar Scheme Shares, and that the Court Meeting will be held at the offices of Nightstar’s solicitors, Skadden, Arps, Slate, Meagher & Flom (UK) LLP, at 40 Bank Street, London E14 5DS on 8 May 2019 at 10.40 a.m. (or as soon thereafter as the General Meeting has been concluded or adjourned) at which place and time all Nightstar Scheme Shareholders (as defined in the Scheme of Arrangement) are requested to attend.

A copy of the Scheme of Arrangement and a copy of the Explanatory Statement required to be furnished pursuant to Part 26 of the Companies Act 2006 are incorporated in the document of which this Notice forms part.

Voting on the resolution to approve the Scheme of Arrangement will be by poll, which shall be conducted as the Chairman of the Court Meeting may determine.

Nightstar Scheme Shareholders entitled to attend and vote at the Court Meeting may vote in person at the Court Meeting or they may appoint another person or persons, whether or not a member of Nightstar, as their proxy or proxies to attend and vote in their stead.

A blue Form of Proxy for use in connection with the Court Meeting is enclosed with this Notice or shall be sent in a separate mailing to those Nightstar Scheme Shareholders who have elected or are deemed to have elected to receive documents and notices from the Company via Nightstar’s website. Nightstar Scheme Shareholders entitled to attend and vote at the meeting, who hold their shares through CREST, may appoint a proxy using the CREST Electronic Proxy Appointment Service.

CREST members who wish to appoint a proxy or proxies through the CREST Electronic Proxy Appointment Service may do so for the Court Meeting and any adjournment(s) thereof by using the procedures described in the CREST Manual.

In order for a proxy appointment or instruction made using the CREST service to be valid, the appropriate CREST message (a “CREST Proxy Instruction”) must be properly authenticated in accordance with the specifications of Euroclear UK & Ireland Limited (“Euroclear”) and must contain the information required for such instructions, as described in the CREST Manual. The message, regardless of whether it relates to the

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appointment of a proxy or to an amendment to the instruction given to a previously appointed proxy must, in order to be valid, be transmitted so as to be received by the Company’s registrars, Computershare, (under CREST participant ID 3RA50) by 10.40 a.m. on 3 May 2019 or in the case of any adjournment, not later than 48 hours before the time appointed for the adjourned Court Meeting. For this purpose, the time of receipt will be taken to be the time (as determined by the time stamp applied to the message by the CREST Applications Host) from which Computershare is able to retrieve the message by enquiry to CREST in the manner prescribed by CREST.

Nightstar may treat as invalid a CREST Proxy Instruction in the circumstances set out in Regulation 35(5)(a) of the Uncertificated Securities Regulations 2001.

Completion and return of a blue Form of Proxy will not prevent a Nightstar Scheme Shareholder from attending and voting in person at the Court Meeting or at any adjournment thereof.

In the case of joint holders, the vote of the Nightstar Scheme Shareholder whose name is first listed in the register of members of Nightstar in respect of the joint holding will be accepted to the exclusion of the votes of the other joint holders.

Nightstar Scheme Shareholders are entitled to appoint a proxy in respect of some or all of their shares. Nightstar Scheme Shareholders are also entitled to appoint more than one proxy, provided that each proxy is appointed to exercise the rights attached to a different share or shares held by such Nightstar Scheme Shareholder. A space has been included in the blue Form of Proxy to allow Nightstar Scheme Shareholders to specify the number of shares in respect of which that proxy is appointed. Nightstar Scheme Shareholders who return the blue Form of Proxy duly executed but leave this space blank shall be deemed to have appointed the proxy in respect of all their Nightstar Scheme Shares.

Nightstar Scheme Shareholders who wish to appoint more than one proxy in respect of their shareholding should contact the Company’s registrars, Computershare at The Pavilions, Bridgwater Road, Bristol, BS13 8AE, United Kingdom, for further blue Forms of Proxy, or photocopy the blue Form of Proxy as required. Such Nightstar Scheme Shareholders should also read the information regarding the appointment of multiple proxies set out on page xxi of the document of which this Notice forms part and on the blue Form of Proxy.

As an alternative to appointing a proxy, any Nightstar Scheme Shareholder which is a corporation may appoint one or more corporate representatives who may exercise on its behalf, all of its powers as a member, provided that they do not do so in relation to the same shares. Only one corporate representative is to be counted in determining whether under section 899(1) of the Companies Act 2006 a majority in number of the Nightstar Scheme Shareholders approved the Scheme of Arrangement. The Chairman of the Court Meeting may require a corporate representative to produce to the Company’s registrars, Computershare, his or her written authority to attend and vote at the Court Meeting at any time before the start of the Court Meeting. The representative shall not be entitled to exercise the powers conferred on them by the Nightstar Scheme Shareholder until any such demand has been satisfied.

It is requested that blue Forms of Proxy be returned using the prepaid envelope provided to Computershare at The Pavilions, Bridgwater Road, Bristol, BS13 8AE, United Kingdom by 10.40 a.m. on 3 May 2019 or in the case of any adjournment, not later than 48 hours (excluding any part of a day that is not a working day) before the time appointed for the adjourned Court Meeting, but if blue Forms of Proxy are not so returned they may be handed to the Chairman of the Court Meeting at the commencement of the Court Meeting.

Only those Nightstar Scheme Shareholders registered in the register of members of Nightstar as at 6.00 p.m. on 3 May 2019 or, in the event that the Court Meeting is adjourned, in the register of members at 6.00 p.m. 48 hours (excluding any part of a day that is not a working day) before the day of any adjourned meeting shall be entitled to attend or vote in respect of the number of shares registered in their name at the relevant time. Changes

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to entries in the relevant register of members after 6.00 p.m. on 3 May 2019 or, in the event that the Court Meeting is adjourned, after 6.00 p.m. 48 hours (excluding any part of a day that is not a working day) before the day of any adjourned meeting shall be disregarded in determining the rights of any person to attend or vote at the Court Meeting. Nightstar Shareholders who hold their Nightstar Scheme Shares in the name of a broker, bank or other nominee should follow the voting instructions provided by such nominee to ensure that their Nightstar Scheme Shares are represented at the Court Meeting.

By the said order, the Court has appointed Chris Hollowood or, failing him, any other director of the Company to act as Chairman of the Court Meeting and has directed the Chairman to report the result of the Court Meeting to the Court.

The Scheme of Arrangement will be subject to the subsequent approval of the Court.

DATED: 9 April 2019

Skadden, Arps, Slate, Meagher & Flom (UK) LLP

40 Bank Street

London E14 5DS

Solicitors for the Company

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NOTICE OF GENERAL MEETING

NIGHTSTAR THERAPEUTICS PLC

(incorporated in England and Wales with registered number 10852952)

NOTICE IS HEREBY GIVEN that a General Meeting of Nightstar Therapeutics plc, a public limited company incorporated in England and Wales with its registered office at 10 Midford Place, London, W1T 5BJ (the “Company”) will be held at the offices of Nightstar’s solicitors, Skadden, Arps, Slate, Meagher & Flom (UK) LLP, at 40 Bank Street, London E14 5DS on 8 May 2019 at 10.30 a.m. for the purpose of considering and, if thought fit, passing the resolutions set out below, which in the case of Resolutions 1 and 3 will be proposed as a special resolution and Resolution 2 will be proposed as an ordinary resolution.

SPECIAL RESOLUTION

notwithstanding anything contained in the Company’s articles of association, the board of directors of the Company (the “Board”) be authorised to re-designate any ordinary share of GBP 0.01 in the capital of the Company (each, an “Ordinary Share”) that is subject to a vesting agreement dated on or about 27 September 2017, entered into between the Company and certain then employee-holders of Ordinary Shares in the capital of the Company, as a Deferred Share in the capital of the Company, and that the Board may do and take all such action as it may consider necessary or appropriate for ensuring that such re-designation is effected.

ORDINARY RESOLUTION

conditional upon passing Resolution 1 and the Board so authorising, the terms of each of the buy-back agreements to acquire any Deferred Shares in the forms available for inspection on the Company’s website and at its registered office be and is hereby approved.

SPECIAL RESOLUTION

for the purpose of giving effect to the scheme of arrangement dated 9 April 2019 (the “Scheme”) between the Company and the holders of the Nightstar Scheme Shares (as defined in the Scheme), a print of which has been produced to this meeting and for the purposes of identification signed by the Chairman hereof, in its original form or subject to such modification, addition or condition agreed between the Company and Biogen Switzerland Holdings GmbH and approved or imposed by the Court:

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For the purposes of this Article, “Nightstar Share Schemes” means:

each as amended from time to time.

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Notes

The following notes explain your general rights as a shareholder and your rights to attend and vote at the General Meeting or to appoint someone else to vote on your behalf.

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In order for a proxy appointment made by means of CREST to be valid, the appropriate CREST message (a CREST Proxy Instruction) must be properly authenticated in accordance with Euroclear UK & Ireland Limited’s specifications and must contain the information required for such instructions, as described in the CREST Manual. The message must be transmitted so as to be received by Computershare (under CREST participant ID 3RA50) not later than 10.30 a.m. on 3 May 2019. For this purpose, the time of receipt will be taken to be the time (as determined by the timestamp applied to the message by the CREST Applications Host) from which Computershare is able to retrieve the message by enquiry to CREST. After this time, any change of instructions to proxies appointed through CREST should be communicated to the appointee through other means.

CREST members and, where applicable, their CREST sponsors or voting service providers should note that Euroclear UK & Ireland Limited does not make available special procedures in CREST for any particular messages. Normal system timings and limitations will therefore apply in relation to the input of CREST Proxy Instructions. It is the responsibility of the CREST member concerned to take (or, if the CREST member is a CREST personal member or sponsored member or has appointed a voting service provider(s), to procure that his CREST sponsor or voting service provider(s) take(s)) such action as shall be necessary to ensure that a message is transmitted by means of the CREST system by any particular time. In this connection, CREST members and, where applicable, their CREST sponsors or voting service providers are

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referred, in particular, to those sections of the CREST Manual concerning practical limitations of the CREST system and timings.

The Company may treat as invalid a CREST Proxy Instruction in the circumstances set out in Regulation 35(5)(a) of the Uncertificated Securities Regulations 2001.

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EXPECTED TIMETABLE OF PRINCIPAL EVENTS

The expected date of the Court Hearing to sanction the Scheme and each of the other dates and times set out below will depend, among other things, on the date on which the conditions to the Scheme and the Acquisition are satisfied or, if capable of waiver, waived. They are accordingly presented as indicative and referable to the date on which those conditions are satisfied or waived (as the case may be). Further details of the conditions are set out in Part III (Conditions to and Further Terms of the Scheme and the Acquisition) of this document.

Nightstar will give notice of each of the below dates and times, when known, by issuing an announcement through GlobeNewswire or another national news wire service and by making such announcement available on Nightstar’s website https://ir.nightstartx.com. Further updates or changes to other times or dates indicated below shall be notified in the same manner.

The General Meeting and the Court Meeting will each be held at the offices of Nightstar’s solicitors, Skadden, at 40 Bank Street, London E14 5DS, at 10.30 a.m. and 10.40 a.m., respectively, on 8 May 2019.

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All references in this timetable to times are to times in London (unless otherwise stated).

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TO VOTE ON THE PROPOSALS

The General Meeting and the Court Meeting will be held at the offices of Nightstar’s solicitors, Skadden, at 40 Bank Street, London E14 5DS on 8 May 2019 at 10.30 a.m. and 10.40 a.m., respectively (or, in the case of the Court Meeting, as soon thereafter as the General Meeting has been concluded or adjourned). In respect of the General Meeting, you are entitled to one vote for each Nightstar Share that you hold as at the Scheme Voting Record Time. In respect of the Court Meeting, you are entitled to one vote for each Nightstar Scheme Share that you hold as at the Scheme Voting Record Time. As at 5 April 2019 (being the last practicable date prior to the publication of this document), there were 33,536,214 Nightstar Shares issued and outstanding.

Whether or not you plan to attend the Nightstar Shareholder Meetings, if you are a Nightstar Shareholder, please:

so that they are received no later than 10.40 a.m. on 3 May 2019 (in the case of the BLUE Form of Proxy for the Court Meeting) or 10.30 a.m. on 3 May 2019 (in the case of the WHITE Form of Proxy for the General Meeting). A prepaid envelope is provided for this purpose in the U.K. only. Your vote will be cast as specified on the applicable Form of Proxy.

Alternatively, BLUE Forms of Proxy (but NOT WHITE Forms of Proxy) may be handed to a representative of Computershare, on behalf of the Chairman of the Court Meeting, at the venue of the Court Meeting or the Chairman of the Court Meeting before the start of the Court Meeting on 8 May 2019. In the case of the General Meeting, unless the WHITE Form of Proxy is returned by the time and date mentioned in the instructions printed thereon, it will be invalid.

The completion and return of the Forms of Proxy will not prevent eligible Nightstar Shareholders from attending and voting at the Court Meeting or the General Meeting, or any adjournment thereof, in person.

Nightstar Shareholders who hold Nightstar Shares in CREST may appoint a proxy or proxies through the CREST Electronic Proxy Appointment Service using the procedures described in the CREST Manual.

Nightstar Shareholders who prefer to register the appointment of their proxy electronically via the internet can do so through www.eproxyappointment.com using their shareholder investor code found on their Form of Proxy and following the online instructions. Further details in relation to electronic appointment of proxies are set out on page xxii of this document.

IT IS IMPORTANT THAT, FOR THE COURT MEETING, AS MANY VOTES AS POSSIBLE ARE CAST SO THAT THE COURT MAY BE SATISFIED THAT THERE IS A FAIR REPRESENTATION OF NIGHTSTAR SHAREHOLDER OPINION. YOU ARE THEREFORE STRONGLY URGED TO SIGN THE FORMS OF PROXY AND RETURN THEM BY POST USING THE PREPAID ENVELOPE PROVIDED, DURING NORMAL BUSINESS HOURS ONLY, TO COMPUTERSHARE AT THE PAVILIONS, BRIDGWATER ROAD, BRISTOL, BS13 8AE, UNITED KINGDOM, AS SOON AS POSSIBLE AND, IN ANY EVENT, SO AS TO BE RECEIVED BY COMPUTERSHARE PRIOR TO THE DEADLINES SET OUT ABOVE, OR IN THE CASE OF AN ADJOURNED MEETING, NOT LESS THAN 48 HOURS PRIOR TO THE TIME AND DATE SET FOR THE ADJOURNED MEETING.

IF YOU ARE A NIGHTSTAR SHAREHOLDER, YOUR ATTENTION IS DRAWN TO THE REGIME FOR THE APPOINTMENT OF PROXIES SET OUT IN THE NOTES TO THE FORMS OF PROXY AND THE NOTES SET OUT IN THE NOTICE OF COURT MEETING AND THE NOTICE OF THE GENERAL MEETING, INCLUDING IN RESPECT OF THE APPOINTMENT OF MULTIPLE PROXIES.

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This page should be read in conjunction with the section entitled “ACTION TO BE TAKEN” on pages xviii and xix of this document and the section entitled “FORM OF PROXY FOR VOTING AT THE COURT MEETING AND GENERAL MEETING” on page xx of this document, the rest of this document and the accompanying Forms of Proxy.

Nightstar Shareholder Helpline

If you have any queries relating to this document or the completion and return of the Forms of Proxy, please call the Nightstar Shareholder Helpline at +44 370 703 6033. Lines are open Monday to Friday (except public holidays) between 8.30 a.m. and 5.30 p.m.

Nightstar ADS Holders who have any questions should contact the Nightstar Depositary using the contact details provided on the ADS Voting Instruction Card provided to you or found at www.citi.com/dr. If you hold Nightstar ADSs indirectly, you should contact the bank, broker, financial institution or administrator through which you hold Nightstar ADSs.

Calls will be charged at the standard geographic rate and will vary by provider. International calls will be charged at the applicable international rate. Different charges may apply to calls from mobile telephones and calls may be recorded and randomly monitored for security and training purposes. Please note that the Nightstar Shareholder Helpline operators cannot provide advice on the merits of the Scheme or the Acquisition or give any financial, legal, investment or tax advice.

Proposals

At the Court Meeting you will be asked to approve the Scheme, with or subject to any modification, addition or condition which Nightstar and Bidder may agree and which the Court may approve or impose, pursuant to which the Nightstar Scheme Shares will be acquired by Bidco.

At the General Meeting you will be asked to approve resolutions (i) to authorise the Nightstar Board to re-designate any Nightstar Share as a Deferred Share in respect of individuals who have entered into a Vesting Agreement with Nightstar, (ii) to approve, conditional upon the passing of the preceding resolution, the terms of each of the Buy-Back Agreements available for inspection on Nightstar’s website and at its registered office, and in a form substantially identical to Annex E, and (iii) to authorise the Nightstar Directors to take all such action as they may consider necessary or appropriate for carrying the Scheme into effect, and to amend, with effect from the passing of such resolution, the Nightstar Articles to ensure that any Nightstar Shares issued after the General Meeting will be subject to the Scheme or otherwise transferred to Bidco.

Nightstar Scheme Shareholders who currently hold Nightstar Shares which are proposed to be re-designated as Deferred Shares under the terms of a Vesting Agreement will not (subject to the passing of the Re-designation Special Resolution) be entitled to vote on the Ordinary Resolution or the Scheme Special Resolution.

Recommendation and Voting by Nightstar Directors

For the reasons set out in this document, the Nightstar Directors, who have been so advised by Centerview as to the financial terms of the Acquisition, consider the terms of the Acquisition to be fair and reasonable. In providing advice to the Nightstar Directors, Centerview has taken into account the commercial assessments of the Nightstar Directors. Centerview’s advice to the Nightstar Directors is set out in its written opinion, dated 3 March 2019, attached hereto as Annex B and which also sets out in full the relevant assumptions and qualifications to such advice.

The Nightstar Directors believe that the terms of the Acquisition, including the terms of the Implementation Agreement, are fair and reasonable and that proceeding with the Acquisition is likely to promote the success

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of Nightstar for the benefit of its shareholders, having had regard to the interests of other stakeholders, and unanimously recommend that you vote in favour of the Scheme at the Court Meeting and the Resolutions to be proposed at the General Meeting, as the Nightstar Directors have irrevocably undertaken to do, or to procure, in respect of their own beneficial holdings of Nightstar Shares amounting to, in aggregate, 758,019 Nightstar Shares representing approximately 2.3 per cent. of the existing issued ordinary share capital of Nightstar on 5 April 2019 (being the last practicable date prior to the publication of this document). Further details of these Irrevocable Undertakings are contained in paragraph 7 of Part V (Additional Information) of this document.

ACTION TO BE TAKEN / GENERAL INFORMATION

The Scheme requires approval at a meeting of the Nightstar Scheme Shareholders convened with the permission of the Court (known as the Court Meeting) and at the General Meeting. The General Meeting and the Court Meeting will be held at the offices of Nightstar’s solicitors, Skadden, at 40 Bank Street, London E14 5DS on 8 May 2019 at 10.30 a.m. and 10.40 a.m., respectively (or, in the case of the Court Meeting, as soon thereafter as the General Meeting has been concluded or adjourned).

Please check you have received the following with this document:

All Nightstar Shareholders:

If you have not received these documents, please contact Nightstar’s registrars, Computershare, on the Nightstar Shareholder Helpline referred to on page xvii of this document, or your broker, bank or other nominee.

All Nightstar ADS Holders:

If you have not received these documents, please contact the Nightstar Depositary’s ADS shareholder services at +1-877-248-4237, or your broker, bank or other nominee.

Solicitation of Proxies

The Nightstar Directors are soliciting your proxy, and Nightstar will bear the cost of this solicitation of proxies. This includes the charges and expenses of brokerage firms and others for forwarding solicitation material to Nightstar Shareholders.

Nightstar has retained MacKenzie Partners, Inc., a proxy solicitation firm, to assist the Nightstar Directors in the solicitation of proxies for the Court Meeting and the General Meeting. Nightstar expects to pay MacKenzie Partners, Inc., approximately USD 20,000, plus reimbursement of out-of-pocket expenses. Proxies may be solicited by mail, personal interview, e-mail, telephone, or via the internet by MacKenzie Partners, Inc. or, without additional compensation, by certain Nightstar Directors, officers and employees.

Shareholder Proposals

Under English law, except as provided below, there is no general right for a shareholder of a U.K. public limited company to put items on the agenda of a general meeting (save for the annual general meeting) that has been

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convened by directors. English law provides that shareholders holding not less than 5 per cent. of Nightstar’s paid up share capital carrying voting rights may requisition the Nightstar Board to convene a general meeting and may require resolutions to be put before a general meeting they have convened.

If the Acquisition does not become Effective, Nightstar Shareholders will be entitled to attend and participate in Nightstar’s annual general meeting. Under the Companies Act 2006, Nightstar is required to hold its 2019 annual general meeting by no later than 30 June 2019. If any such annual general meeting is held, shareholder proposals will be eligible for consideration for inclusion in the proxy statement and form of proxy for such annual general meeting in accordance with Rule 14a-8 under the U.S. Exchange Act and the Nightstar Articles.

Because the 2019 annual general meeting would be Nightstar’s first annual general meeting, pursuant to Rule 14a-8 under the U.S. Exchange Act, Nightstar may set a deadline for receipt of Rule 14a-8 shareholder proposals that is a reasonable time before Nightstar plans to print and send its proxy materials. Nightstar Shareholders who wish to have a proposal considered for inclusion in Nightstar’s proxy materials for its 2019 annual general meeting pursuant to Rule 14a-8 must ensure that their proposal is received at Nightstar’s registered office by 30 April 2019, which Nightstar has determined to be a reasonable time before it expects to begin to print and send its proxy materials. The 30 April 2019 deadline will also apply in determining whether notice of a shareholder proposal is timely for purposes of exercising discretionary voting authority with respect to proxies under Rule 14a-4(c) under the U.S. Exchange Act.

In addition to the process under the U.S. Exchange Act described above, under the Companies Act 2006, shareholders representing at least 5 per cent. of the total voting rights of all shareholders who have a right to vote at the 2019 annual general meeting of Nightstar can require Nightstar to give Nightstar Shareholders notice of a resolution which may be and is intended to be moved at the 2019 annual general meeting of Nightstar unless (a) the resolution would, if passed, be ineffective (whether by reason of inconsistency with any enactment or Nightstar’s Articles or otherwise); (b) it is defamatory of any person; or (c) it is frivolous or vexatious. Such a request, made by the requisite number of Nightstar Shareholders, must be received by Nightstar not later than six weeks before the annual general meeting.

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FORM OF PROXY FOR VOTING AT THE COURT MEETING

AND GENERAL MEETING

It is important that, for the Court Meeting, as many votes as possible are cast so that the Court may be satisfied that there is a fair representation of the opinion of Nightstar Scheme Shareholders. Therefore, whether or not you plan to attend the Nightstar Shareholder Meetings, please complete and sign both the enclosed BLUE and WHITE Forms of Proxy, or deliver your voting instructions by one of the other methods mentioned below, as soon as possible. Your vote will be cast as specified on the applicable Form of Proxy.

Sending Forms of Proxy by post

Nightstar Shareholders will find enclosed with this document a BLUE Form of Proxy for use in connection with the Court Meeting and a WHITE Form of Proxy for use in connection with the General Meeting. Please complete and sign the enclosed Forms of Proxy in accordance with the instructions printed thereon and return them by post using the prepaid envelope provided to Computershare at The Pavilions, Bridgwater Road, Bristol, BS13 8AE, United Kingdom, so as to be received as soon as possible and in any event not later than:

or, in the case of either meeting being adjourned, not later than 48 hours before the time fixed for the holding of the adjourned meeting. A prepaid envelope is provided for this purpose in the U.K. only, Forms of Proxy returned by fax will not be accepted. This will enable your votes to be counted at the Nightstar Shareholder Meetings in the event of your absence.

If the BLUE Form of Proxy for use at the Court Meeting is not returned by such time, it may be handed to the Chairman of the Court Meeting or a representative of Nightstar’s registrars, Computershare, on behalf of the Chairman of the Court Meeting, before the start of that meeting. However, in the case of the General Meeting, the WHITE Form of Proxy must be received by the time mentioned above, or it will be invalid.

Proxy appointment

Nightstar Shareholders entitled to attend and vote at the Court Meeting and the General Meeting may appoint a proxy to attend and to speak and vote in his/her place.

Nightstar Shareholders are entitled to appoint a proxy in respect of some or all of their Nightstar Shares and may also appoint more than one proxy, provided that each proxy is appointed to exercise the rights attached to a different share or shares held by such holder. A proxy need not be a member of Nightstar.

Please note that the appointment of a proxy or proxies is separate for each of the Court Meeting and the General Meeting.

To be valid, Forms of Proxy must be executed by or on behalf of the Nightstar Shareholder or, if you are a corporate, under the hand of a duly authorised officer or attorney.

If two or more valid, but differing, appointments of proxy are delivered or received in respect of the same share, the one which is last validly delivered or received (regardless of its date or of the date of its execution) shall be treated as replacing and revoking the other or others as regards that share. If Nightstar is unable to determine which instrument was last validly delivered or received, none of them shall be treated as valid in respect of that share.

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The completion and return of the Forms of Proxy, or the appointment of a proxy electronically using CREST (or any other procedure listed below), will not prevent Nightstar Shareholders from attending and voting in person at the Court Meeting and/or the General Meeting, or any adjournment thereof, if you wish and are entitled to do so. Shareholders who hold their Nightstar Shares in the name of a broker, bank or other nominee should follow the voting instructions provided by such nominee to ensure that their Nightstar Shares are represented at the Court Meeting and the General Meeting.

Revocability of proxies

Any person submitting a Form of Proxy pursuant to this solicitation has the power to revoke and change it at any time before it is voted. If you are a Nightstar Shareholder of record, you may revoke your proxy at any time before the vote is taken at the Court Meeting or the General Meeting, as applicable, by:

Please note, however, that only your last validly delivered or received proxy will count (regardless of its date or of the date of its execution). Attending the Court Meeting or General Meeting without taking one of the actions described above will not in itself revoke your proxy.

If you hold your shares indirectly through a broker, bank or other nominee, you will need to follow the instructions provided to you by your broker, bank or other nominee in order to revoke your proxy or submit new voting instructions. If you hold your shares indirectly, you may also revoke a prior proxy by voting in person at the Court Meeting and/or General Meeting if you obtain a Form of Proxy executed in your favour from your broker, bank or other nominee in order to be able to vote in person at such meeting.

Effect of abstentions and broker non-votes

An abstention occurs when a shareholder abstains from voting (either in person or by proxy) on one or more of the proposals. Broker non-votes occur when a broker, bank, trust or other nominee returns a proxy but does not have authority to vote on a particular proposal.

In connection with the Court Meeting, abstentions and broker non-votes will not be considered votes cast and will therefore not have any effect on the outcome of the vote at the Court Meeting.

In connection with the General Meeting, abstentions and broker non-votes will be considered in determining the presence of a quorum. However, abstentions and broker non-votes are not considered votes cast and therefore will not have any effect on the outcome of the vote for the purposes of determining whether the Resolutions have been approved.

Multiple proxy voting instructions

Nightstar Shareholders who wish to appoint more than one proxy in respect of their shareholding should photocopy the Form of Proxy or call the Nightstar Shareholder Helpline on the telephone number set out on page xvii of this document. You should also indicate by ticking the box provided if the proxy is one of multiple instructions being given, fill in the name of the proxy and the number of shares in respect of which the proxy is appointed and return the multiple forms together (please ensure that all of the multiple Forms of Proxy in respect of one registered holding are sent in the same envelope if possible) by the time and date mentioned above.

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Online appointment of proxies

As an alternative to completing and returning the printed Forms of Proxy, Forms of Proxy may be submitted electronically by logging on to the following website: www.eproxyappointment.com and following the instructions there. For an electronic proxy appointment to be valid, the appointment must be received by Computershare no later than:

or, in the case of either meeting being adjourned, no later than 48 hours before the time fixed for the holding of the adjourned meeting. In order to access the voting system, Nightstar Shareholders will need their shareholder investor code found on their Form of Proxy.

In the case of the Court Meeting only, if you have not appointed a proxy electronically by such time you may complete the BLUE Form of Proxy and hand it to the Chairman of the Court Meeting or a representative of Computershare, on behalf of the Chairman of the Court Meeting, before the start of that meeting.

Electronic appointment of proxies through CREST

If you hold your Nightstar Shares in uncertificated form through CREST and wish to appoint a proxy or proxies for the Nightstar Shareholder Meetings (or any adjourned meeting) by using the CREST Electronic Proxy Appointment Service, you may do so using the procedures described in the CREST Manual (available at www.euroclear.com). CREST personal members or other CREST sponsored members, and those CREST members who have appointed any voting service provider(s), should refer to their CREST sponsor or voting service providers, who will be able to take the appropriate action on their behalf.

In order for a proxy appointment or instruction made using the CREST service to be valid, the CREST Proxy Instruction must be properly authenticated in accordance with Euroclear’s specifications and must contain the information required for such instructions, as described in the CREST Manual. The message, regardless of whether it constitutes the appointment of a proxy or an amendment to the instruction given to a previously appointed proxy must, in order to be valid, be transmitted so as to be received by Computershare (under CREST participant ID 3RA50) not later than 10.30 a.m. on 3 May 2019 in the case of the General Meeting and not later than 10.40 a.m. on 3 May 2019 in the case of the Court Meeting or, in the case of any adjournment, not later than 48 hours before the time fixed for the holding of the adjourned meeting. For this purpose, the time of receipt will be taken to be the time (as determined by the time stamp applied to the message by the CREST Applications Host) from which Computershare is able to retrieve the message by enquiry to CREST in the manner prescribed by CREST. After this time, any change of instructions to proxies appointed through CREST should be communicated to the appointee through other means.

CREST members and, where applicable, their CREST sponsors or voting service providers should note that Euroclear does not make available special procedure(s) in CREST for any particular messages. Normal system timings and limitations will therefore apply in relation to the input of CREST Proxy Instructions. It is the responsibility of the CREST member concerned to take (or, if the CREST member is a CREST personal member or sponsored member or has appointed one or more voting service providers, to procure that his or her CREST sponsor or voting service provider(s) take(s)) such action as shall be necessary to ensure that a message is transmitted by means of the CREST system by any particular time. In this connection, CREST members and, where applicable, their CREST sponsors or voting service provider(s) are referred, in particular, to those sections of the CREST Manual concerning practical limitations of the CREST system and timings. Nightstar may treat as invalid a CREST Proxy Instruction in the circumstances set out in Regulation 35(5)(a) of the Uncertificated Securities Regulations 2001.

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Further information about proxies and voting

Further information in relation to the appointment of proxies for and voting at the Nightstar Shareholder Meetings is set out in paragraph 19 of Part II (Explanatory Statement) of this document, in the notice of the Court Meeting set out on pages vi to viii of this document (Notice of Court Meeting), in the notes to the notice of the General Meeting set out on pages ix to xiii of this document (Notice of General Meeting), and in the instructions printed on the Forms of Proxy.

If you hold Nightstar Shares indirectly, you must rely on the procedures of the bank, broker, financial institution, share plan administrator or share plan nominee or other securities intermediary through which you hold Nightstar Shares. You should contact such intermediary for further instructions on how you can instruct that intermediary to vote on your behalf at the Nightstar Shareholder Meetings and the date by which you must provide such instructions to the intermediary.

If the Scheme becomes Effective, it will be binding on all Nightstar Scheme Shareholders, including any Nightstar Shareholders who did not vote to approve the Scheme or who voted against the Scheme at the Court Meeting or against the Resolutions at the General Meeting.

No “appraisal” rights

No Nightstar Shareholders or Nightstar ADS Holders will have “appraisal” or “dissenters” rights or otherwise have any right to seek an appraisal of the Nightstar Shares.

Nightstar Share Schemes

Participants in the Nightstar Share Schemes should refer to paragraph 9 of Part II (Explanatory Statement) of this document for information relating to the effect of the Acquisition on their rights under the Nightstar Share Schemes.

Nightstar ADS Holders

Nightstar ADS Holders should refer to paragraph 15 of Part II (Explanatory Statement) of this document.

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CONTENTS

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QUESTIONS AND ANSWERS ABOUT THE NIGHTSTAR SHAREHOLDER MEETINGS AND THE ACQUISITION

The following questions and answers address briefly some questions you may have regarding the Nightstar Shareholder Meetings and the proposals to be voted on at the Nightstar Shareholder Meetings. These questions and answers may not address all of the questions that may be important to you as a Nightstar Shareholder. Please refer to the more detailed information contained elsewhere in this document, the annexes to this document and the documents referred to or incorporated by reference in this document, which you should read carefully and in their entirety. You may obtain the information incorporated by reference into this document without charge by following the instructions under the section entitled “Documents” in paragraph 13 of Part V (Additional Information).

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If you hold your Nightstar Shares through a broker, bank or other nominee at the Scheme Voting Record Time you will need to contact your broker, bank or other nominee in order to submit your voting instructions.

If you hold Nightstar Shares registered in your own name, you are entitled, subject to compliance with procedures described below, to vote either in person or by appointing another person or persons as your proxy or proxies to attend and vote in your stead.

At the General Meeting, conditional upon passing the Re-designation Special Resolution (as defined below), Nightstar Shareholders will vote on an ordinary resolution to approve the terms of each of the Buy-Back Agreements available for inspection on Nightstar’s website and at its registered office and in a form substantially identical to Annex E. This resolution is referred to as the “Ordinary Resolution.”

Nightstar Scheme Shareholders who currently hold Nightstar Shares which are proposed to be re-designated as Deferred Shares under the terms of a Vesting Agreement will not (subject to the passing of the Re-designation Special Resolution) be entitled to vote on the Ordinary Resolution or the Scheme Special Resolution.

At the General Meeting, Nightstar Shareholders will also vote on special resolutions to (i) authorise the Nightstar Board to re-designate any Nightstar Share as a Deferred Share in respect of individuals who have entered into a Vesting Agreement with Nightstar; this resolution is referred to as the “Re-designation Special Resolution” and (ii) (a) authorise the Nightstar Board to take all necessary action to carry the Scheme into effect and (b) amend the Nightstar Articles to ensure that any Nightstar Shares issued after the Scheme Record Time will be subject to the Scheme or otherwise transferred to Bidco, so that no other person will hold Nightstar Shares following the Effective Date; this resolution is referred to as the “Scheme Special Resolution.”

Under the terms of the Vesting Agreements, the relevant holders agreed that upon the date such individual ceased to hold all positions as director, employee or consultant of any member of the Nightstar Group or ceased to provide services to any member of the Nightstar Group or if earlier the giving or receiving of notice of such cessation, the Nightstar Board would determine, and notify the holder of, the proportion of such holder’s shareholding that would convert to Deferred Shares.

Under Article 10 of the Nightstar Articles, the Deferred Shares are, for all practical purposes, valueless. Nightstar has not at present re-designated any such Nightstar Shares as Deferred Shares. The Nightstar Board is seeking the authority through the Re-designation Special Resolution to re-designate such Nightstar Shares as Deferred Shares as provided in the Vesting Agreements. Nightstar believes that as at the date of this document, 48,721 Nightstar Shares will be re-designated as Deferred Shares under the terms of the Vesting Agreements.

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If you are a shareholder of record, please be prepared to provide proper identification, such as a driver’s license. If you hold your shares through a broker, bank or other nominee you will need to provide proof of ownership, such as a recent account statement provided by your broker, bank or other nominee or other similar evidence of ownership, along with proper identification.

Nightstar ADS Holders will not be able to attend the Court Meeting or the General Meeting in person. If you are a Nightstar ADS Holder as of the Nightstar ADS Voting Record Time and you wish to attend the Court Meeting and the General Meeting, you will need to arrange for the cancellation of your Nightstar ADSs in order to become a Nightstar Shareholder prior to the applicable Scheme Voting Record Time. If you present your Nightstar ADSs for cancellation after the Nightstar ADS Voting Record Time to attend the Court Meeting or the General Meeting, you will need to certify that you were a Nightstar ADS Holder as of the Nightstar ADS Voting Record Time and that you will not vote the Nightstar ADSs previously held.

In connection with the Court Meeting, there must be a sufficiently large (in the Court’s judgment) number of Nightstar Scheme Shares included in the vote to fairly represent the opinion of Nightstar Scheme Shareholders. Abstentions and broker non-votes will not be considered votes cast and will therefore not have any effect on the outcome of the vote at the Court Meeting.

In addition to approval by Nightstar Scheme Shareholders at the Court Meeting, approval of the Scheme will also require sanction by the Court. In considering whether to sanction the Scheme, the Court will consider whether there has been a sufficiently large (in the Court’s judgment) number of Nightstar Scheme Shares included in the vote in favour of the Scheme to fairly represent the opinion of Nightstar Scheme Shareholders, in addition to whether the required majority (as described above), is obtained.

At the General Meeting, the Ordinary Resolution must be approved by Nightstar Shareholders representing more than 50 per cent. of the votes cast and each of the Special Resolutions must be approved by Nightstar Shareholders representing at least 75 per cent. of the votes cast.

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It is currently expected that Nightstar’s executive officers will vote their Nightstar Shares in favour of the Acquisition and the other proposals to be considered at the Nightstar Shareholder Meetings.

If you hold Nightstar Shares registered in your own name, you are entitled to attend the General Meeting to vote in person or to appoint another person or persons as your proxy or proxies to attend and vote in your stead, in accordance with the procedures further outlined in this document.

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If you hold your Nightstar Shares through a broker, bank or other nominee you will need to contact your broker, bank or other nominee in order to submit your voting instructions for both the Court Meeting and the General Meeting.

You are strongly encouraged to sign and return the blue Form of Proxy for the Court Meeting as soon as possible or, if you hold Nightstar Shares in CREST, via a CREST Proxy Instruction. You are also encouraged to sign and return the white Form of Proxy for the General Meeting at the same time as the blue Form of Proxy for the Court Meeting or if you hold Nightstar Shares in CREST, via a CREST Proxy Instruction.

If you are a Nightstar Shareholder of record, you may revoke your proxy at any time before the vote is taken at the Court Meeting or the General Meeting, as applicable, by:

Please note, however, that only your last validly delivered or received proxy will count (regardless of its date or of the date of its execution). Attending the Court Meeting or General Meeting without taking one of the actions described above will not in itself revoke your proxy.

If you hold your shares indirectly through a broker, bank or other nominee, you will need to follow the instructions provided to you by your broker, bank or other nominee in order to revoke your proxy or submit new voting instructions. If you hold your shares indirectly, you may also revoke a prior proxy by voting in person at the Court Meeting and/or General Meeting if you obtain a Form of Proxy executed in your favour from your broker, bank or other nominee in order to be able to vote in person at such meeting.

If your Nightstar Shares are held by a broker, bank or other nominee, you will need to contact your broker, bank or other nominee in order to confirm the answer to this question. However, if you do not instruct your broker, bank or other nominee how to vote your shares, your shares will not be counted for purposes of calculating the proportion of votes “For” and “Against” at the Nightstar Shareholder Meetings. Broker non-votes will be considered in determining the presence of quorum at the General Meeting.

There is a “Vote Withheld” option to enable you to abstain from voting on the Resolutions at the General Meeting. A “Vote Withheld” will count towards quorum at the General Meeting, however a “Vote Withheld” is not a vote in law and will not be counted in the calculation of the proportion of votes “For” and “Against” the Resolutions at the General Meeting.

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If you are a Nightstar ADS Holder as of the Nightstar ADS Voting Record Time and you wish to attend the Court Meeting and the General Meeting, you will need to arrange for the cancellation of your Nightstar ADSs in order to become a Nightstar Scheme Shareholder prior to the applicable Scheme Voting Record Time. If you present your Nightstar ADSs for cancellation after the Nightstar ADS Voting Record Time to attend the Court Meeting or the General Meeting, you will need to certify that you were a Nightstar ADS Holder as of the Nightstar ADS Voting Record Time and that you will not vote the Nightstar ADSs previously held.

The Scheme Voting Record Time, which determines the Nightstar Shareholders entitled to vote at the Nightstar Shareholder Meetings, is earlier than the consummation of the Acquisition. If you transfer your Nightstar Scheme Shares after the Scheme Voting Record Time but prior to the Scheme Record Time, you will retain any rights you hold to vote at the Nightstar Shareholder Meetings but not the right to receive the Consideration.

If you are a Nightstar ADS Holder, in order to receive the consideration to which you are entitled in relation to any Nightstar ADS, you must surrender your Nightstar ADS to the Nightstar Depositary after the Effective Date. Consequently, if you transfer your Nightstar ADSs on or before the Effective Date, you will not be able to present your Nightstar ADSs to the Nightstar Depositary to receive any consideration in relation thereto.

There is no requirement for registered Nightstar Shareholders to take any action with regard to their share certificates. Share certificates in respect of Nightstar Shares will cease to be valid upon the Scheme becoming Effective.

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If you are a Nightstar ADS Holder who holds your Nightstar ADSs in certificated form you will, after the Effective Date, receive a letter of transmittal from the Nightstar Depositary with an explanation on how to surrender the Nightstar ADSs to the Nightstar Depositary. Such Nightstar ADS Holders must sign and return the letter of transmittal, together with their Nightstar ADS certificates, to receive any consideration to which they are entitled.

If you are a Nightstar ADS Holders who holds your Nightstar ADSs in uncertificated form, you will automatically receive any consideration to which they are entitled and do not need to take any further action (unless your broker, bank or other nominee advises that further action is necessary).

If your Nightstar Shares or Nightstar ADSs are held on your behalf by a broker, bank or other nominee, you may receive instructions from your broker, bank or other nominee as to what action, if any, you need to take to effect the surrender of your Nightstar Shares or Nightstar ADSs in exchange for the Consideration.

If your broker, bank or other nominee holds your shares, you may also call your broker, bank or other nominee for additional information.

ADS Holders may contact the Nightstar Depositary’s ADS shareholder services at +1-877-248-4237 or MacKenzie Partners, Inc., the proxy solicitor for the Nightstar ADSs toll-free at 1-800-322-2885, or +1-212-929-5500 and by email at [email protected].

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PART I

LETTER FROM THE CHAIRMAN OF NIGHTSTAR THERAPEUTICS PLC

(incorporated in England and Wales with registered number 10852952)

To all Nightstar Shareholders and, for information only, to participants in the Nightstar Share Schemes and persons with information rights

Dear Shareholder

RECOMMENDED ACQUISITION

of

NIGHTSTAR THERAPEUTICS PLC

by

TUNGSTEN BIDCO LIMITED

a wholly-owned subsidiary of

BIOGEN SWITZERLAND HOLDINGS GMBH

On 4 March 2019, the Nightstar Board and Biogen announced that they had agreed to the terms of a recommended cash Acquisition of Nightstar pursuant to which Bidco will acquire the entire issued and to be issued share capital of Nightstar. The Acquisition is to be effected by means of a scheme of arrangement under Part 26 of the Companies Act 2006, which requires the approval of Nightstar Shareholders and the sanction of the Court. The Acquisition remains subject to the terms and conditions set out in Part III (Conditions to and Further Terms of the Scheme and the Acquisition) of this document.

The Acquisition is to be effected by way of a Court-sanctioned scheme of arrangement under Part 26 of the Companies Act 2006, which requires the approval of Nightstar Scheme Shareholders at the Court Meeting, the passing of the Resolutions at the General Meeting and the sanction of the Court.

In accordance with the terms of the Acquisition, which is subject to the Conditions and further terms set out in Part III (Conditions to and Further Terms of the Scheme and the Acquisition) of this document, Nightstar Scheme Shareholders at the Scheme Record Time will be entitled to receive:

For each Nightstar Scheme Share,                     USD 25.50 in cash

Based on the exchange rate of approximately USD 1.3: GBP 1 at 4.00 p.m. on 5 April 2019 (being the last practicable date prior to the date of this document):

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The Nightstar Shares will be acquired by Bidco with full title guarantee, fully paid and free from all liens, equitable interests, charges, encumbrances, rights of pre-emption and any other third party rights or interests whatsoever and together with all rights existing at the date of this document or thereafter attaching thereto, including (without limitation) the right to receive and retain, in full, all dividends and other distributions (if any) declared, made or paid or any other return of capital (whether by way of reduction of share capital or share premium account or otherwise) made on or after the Effective Date in respect of the Nightstar Shares.

The Acquisition remains subject to the Conditions and further terms set out in full in Part III (Conditions to and Further Terms of the Scheme and the Acquisition) of this document.

Amongst others, the Conditions include the expiration or early termination of the applicable waiting period under the U.S. HSR Act and the receipt of any necessary approval from the German antitrust authority.

The Scheme is also conditional, amongst other things, upon:

At this stage, subject to the approval and availability of the Court (which is subject to change), and subject to the satisfaction (or, where applicable, waiver) of the Conditions, Bidder expects that the Acquisition will become Effective by mid-year 2019, and no earlier than 7 June 2019.

Biogen is focused on discovering, developing, and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases. Nightstar is a gene therapy company focused on developing novel treatments for patients suffering from rare inherited retinal diseases that would otherwise lead to blindness.

Nightstar’s lead asset is NSR-REP1 for the treatment of choroideremia (CHM), a rare, degenerative, X-linked inherited retinal disorder, which leads to blindness and has no approved treatments. CHM primarily affects males and is caused by loss of function in the CHM gene which encodes the Rab escort protein-1 (REP-1). The REP-1 protein plays a role in intracellular protein trafficking, and loss of function in the CHM gene leads to abnormal intracellular protein trafficking and impaired elimination of waste products from the retinal pigment epithelium and photoreceptors. Initially, patients with CHM experience poor night vision, and over time progressive visual loss ultimately leads to complete blindness.

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NSR-REP1 is comprised of an AAV vector administered by subretinal injection which provides a functioning CHM gene and expression of the REP-1 protein to restore membrane trafficking and thereby slow, stop, or potentially reverse the decline in vision. Data from the Phase 1/2 trial of NSR-REP1 demonstrated potentially meaningful slowing of decline in visual acuity as compared to natural history as well as signs of improved visual acuity in some patients. NSR-REP1 is currently being evaluated in the ongoing Phase 3 STAR trial with data expected in the second half of 2020.

Nightstar’s second clinical program is NSR-RPGR for the treatment of X-linked retinitis pigmentosa (“XLRP”), which is also a rare inherited retinal disease primarily affecting males with no approved treatments. XLRP is characterised by mutations in the retinitis pigmentosa GTPase regulator (“RPGR”) gene leading to a lack of active protein transport in photoreceptors. This abnormality leads to loss of the photoreceptor cells, resulting in retinal dysfunction by adolescence and early adulthood, progressing to legal blindness when patients are in their 40s.

NSR-RPGR is comprised of an AAV vector administered by subretinal injection which provides a functioning RPGR gene and thus expression of the RPGR protein, which is critical for protein transport in photoreceptors. The restoration of photoreceptor function is intended to slow, stop, or potentially reverse the decline in vision. Phase 1/2 data from the dose escalation portion of the XIRIUS trial for NSR-RPGR demonstrated an increase in central retinal sensitivity. The Phase 2/3 dose expansion portion of the XIRIUS trial is currently ongoing.

Nightstar’s preclinical pipeline includes NSR-ABCA4 for Stargardt disease and potential programs targeting Best vitelliform macular dystrophy (Best disease) and other genetic forms of retinitis pigmentosa.

The proposed acquisition of Nightstar would bolster Biogen’s pipeline and further execute on its strategy to develop and expand a multi-franchise neuroscience pipeline across multiple complementary modalities. Ophthalmology is an emerging growth area for Biogen, and the proposed acquisition of Nightstar would accelerate Biogen’s entry into ophthalmology by adding two mid- to late-stage gene therapy assets to Biogen’s pipeline, with the potential to create long-term shareholder value.

For the reasons set out in this document, the Nightstar Directors, who have been so advised by Centerview as to the financial terms of the Acquisition, consider the terms of the Acquisition to be fair and reasonable. In providing advice to the Nightstar Directors, Centerview has taken into account the commercial assessments of the Nightstar Directors. Centerview’s advice to the Nightstar Directors is set out in its written opinion, dated 3 March 2019, attached hereto as Annex B and which also sets out in full the relevant assumptions and qualifications to such advice.

Accordingly, the Nightstar Directors believe that the terms of the Acquisition, including the terms of the Implementation Agreement, are fair and reasonable and that proceeding with the Acquisition is likely to promote the success of Nightstar for the benefit of its shareholders, having had regard to the interests of other stakeholders and unanimously recommend that Nightstar Shareholders vote in favour of the Scheme at the Court Meeting and the Resolutions to be proposed at the General Meeting, as they have irrevocably undertaken to do in respect of their own holdings of, in aggregate, 758,019 Nightstar Shares (representing approximately 2.3 per cent. of the issued ordinary share capital of Nightstar on 5 April 2019 (being the last practicable date prior to the publication of this document)).

On 3 March 2019, Centerview rendered to the Nightstar Board its oral opinion, subsequently confirmed in a written opinion dated such date, that, as of such date and based upon and subject to various assumptions made, procedures followed, matters considered, and qualifications and limitations upon the review undertaken by Centerview in preparing its opinion, the Consideration to be paid to the holders of the Nightstar Shares (other than Excluded Shares) pursuant to the Implementation Agreement was fair, from a financial point of view, to such holders.

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The full text of Centerview’s written opinion, dated 3 March 2019, which describes the assumptions made, procedures followed, matters considered, and qualifications and limitations upon the review undertaken by Centerview in preparing its opinion, is attached hereto as Annex B and is incorporated herein by reference. The summary of the written opinion of Centerview set forth below is qualified in its entirety to the full text of Centerview’s written opinion attached as Annex B. Centerview’s financial advisory services and opinion were provided for the information and assistance of the Nightstar Board (in their capacity as directors and not in any other capacity) in connection with and for purposes of its consideration of the Acquisition and Centerview’s opinion only addressed the fairness, from a financial point of view, as of the date thereof, to the holders of the Nightstar Shares (other than Excluded Shares) of the Consideration to be paid to such holders pursuant to the Implementation Agreement. Centerview’s opinion did not address any other term or aspect of the Implementation Agreement or the Acquisition and does not constitute a recommendation to any Nightstar Shareholder or any other person as to how such shareholder or other person should vote with respect to the Acquisition or the Scheme or whether or not such shareholder should tender their Nightstar Shares in connection with an Offer or any other matter.

The full text of Centerview’s written opinion should be read carefully in its entirety for a description of the assumptions made, procedures followed, matters considered, and qualifications and limitations upon the review undertaken by Centerview in preparing its opinion.

In connection with rendering the opinion described above and performing its related financial analyses, Centerview reviewed, among other things:

Centerview also participated in discussions with members of the senior management and representatives of Nightstar regarding their assessment of the Internal Data. In addition, Centerview reviewed publicly available financial and stock market data, including valuation multiples, for Nightstar and compared that data with similar data for certain other companies, the securities of which are publicly traded, in lines of business that Centerview deemed relevant. Centerview also compared certain of the proposed financial terms of the Acquisition with the financial terms, to the extent publicly available, of certain other transactions that Centerview deemed relevant, and conducted such other financial studies and analyses and took into account such other information as Centerview deemed appropriate.

Centerview assumed, without independent verification or any responsibility therefor, the accuracy and completeness of the financial, legal, regulatory, tax, accounting and other information supplied to, discussed

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with, or reviewed by Centerview for purposes of its opinion and, with Nightstar’s consent, Centerview relied upon such information as being complete and accurate. In that regard, Centerview assumed, at Nightstar’s direction, that the Internal Data (including, without limitation, the Forecasts) were reasonably prepared on bases reflecting the best currently available estimates and judgments of the management of Nightstar as to the matters covered thereby and Centerview relied, at Nightstar’s direction, on the Internal Data for purposes of Centerview’s analysis and opinion. Centerview expressed no view or opinion as to the Internal Data or the assumptions on which it was based. In addition, at Nightstar’s direction, Centerview did not make any independent evaluation or appraisal of any of the assets or liabilities (contingent, derivative, off-balance-sheet or otherwise) of Nightstar, nor was Centerview furnished with any such evaluation or appraisal, and was not asked to conduct, and did not conduct, a physical inspection of the properties or assets of Nightstar. Centerview assumed, at Nightstar’s direction, that the final executed Implementation Agreement would not differ in any respect material to Centerview’s analysis or opinion from the Draft Implementation Agreement reviewed by Centerview. Centerview also assumed, at Nightstar’s direction, that the Acquisition will be consummated on the terms set forth in the Implementation Agreement and in accordance with all applicable laws and other relevant documents or requirements, without delay or the waiver, modification or amendment of any term, condition or agreement, the effect of which would be material to Centerview’s analysis or Centerview’s opinion and that, in the course of obtaining the necessary governmental, regulatory and other approvals, consents, releases and waivers for the Acquisition, no delay, limitation, restriction, condition or other change will be imposed, the effect of which would be material to Centerview’s analysis or Centerview’s opinion. Centerview did not evaluate and did not express any opinion as to the solvency or fair value of Nightstar, or the ability of Nightstar to pay its obligations when they come due, or as to the impact of the Acquisition on such matters, under any state, federal or other laws relating to bankruptcy, insolvency or similar matters. Centerview is not a legal, regulatory, tax or accounting advisor, and Centerview expressed no opinion as to any legal, regulatory, tax or accounting matters.

Centerview expressed no view as to, and Centerview’s opinion did not address, Nightstar’s underlying business decision to proceed with or effect the Acquisition, or the relative merits of the Acquisition as compared to any alternative business strategies or transactions that might be available to Nightstar or in which Nightstar might engage. Centerview’s opinion was limited to and addressed only the fairness, from a financial point of view, as of the date of Centerview’s written opinion, to the holders of the Nightstar Shares (other than Excluded Shares) of the Consideration to be paid to such holders pursuant to the Implementation Agreement. For purposes of its opinion, Centerview was not asked to, and Centerview did not, express any view on, and its opinion did not address, any other term or aspect of the Implementation Agreement or the Acquisition, including, without limitation, the structure or form of the Acquisition, or any other agreements or arrangements contemplated by the Implementation Agreement or entered into in connection with or otherwise contemplated by the Acquisition, including, without limitation, the fairness of the Acquisition or any other term or aspect of the Acquisition to, or any consideration to be received in connection therewith by, or the impact of the Acquisition on, the holders of any other class of securities, creditors or other constituencies of Nightstar or any other party. In addition, Centerview expressed no view or opinion as to the fairness (financial or otherwise) of the amount, nature or any other aspect of any compensation to be paid or payable to any of the officers, directors or employees of Nightstar or any party, or class of such persons in connection with the Acquisition, whether relative to the Consideration to be paid to the holders of the Nightstar Shares (other than Excluded Shares) pursuant to the Implementation Agreement or otherwise. Centerview’s opinion was necessarily based on financial, economic, monetary, currency, market and other conditions and circumstances as in effect on, and the information made available to Centerview as of, the date of Centerview’s written opinion, and Centerview does not have any obligation or responsibility to update, revise or reaffirm its opinion based on circumstances, developments or events occurring after the date of Centerview’s written opinion. Centerview’s opinion does not constitute a recommendation to any Nightstar Shareholder or any other person as to how such shareholder or other person should vote with respect to the Acquisition or the Scheme or whether or not such shareholder should tender the Nightstar Shares in connection with an Offer or any other matter. Centerview’s financial advisory services and its written opinion were provided for the information and assistance of the Nightstar Board (in their capacity as

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directors and not in any other capacity) in connection with and for purposes of its consideration of the Acquisition. The issuance of Centerview’s opinion was approved by the Centerview Partners LLC Fairness Opinion Committee.

Summary of Centerview Financial Analysis

The following is a summary of the material financial analyses prepared and reviewed with the Nightstar Board in connection with Centerview’s opinion, dated 3 March 2019. The summary set forth below does not purport to be a complete description of the financial analyses performed or factors considered by, and underlying the opinion of, Centerview, nor does the order of the financial analyses described represent the relative importance or weight given to those financial analyses by Centerview. Centerview may have deemed various assumptions more or less probable than other assumptions, so the reference ranges resulting from any particular portion of the analyses summarised below should not be taken to be Centerview’s view of the actual value of Nightstar. Some of the summaries of the financial analyses set forth below include information presented in tabular format. In order to fully understand the financial analyses, the tables must be read together with the text of each summary, as the tables alone do not constitute a complete description of the financial analyses performed by Centerview. Considering the data in the tables below without considering all financial analyses or factors or the full narrative description of such analyses or factors, including the methodologies and assumptions underlying such analyses or factors, could create a misleading or incomplete view of the processes underlying Centerview’s financial analyses and its opinion. In performing its analyses, Centerview made numerous assumptions with respect to industry performance, general business and economic conditions and other matters, many of which are beyond the control of Nightstar or any other parties to the Acquisition. None of Nightstar, Bidder, Bidco or Centerview or any other person assumes responsibility if future results are materially different from those discussed. Any estimates contained in these analyses are not necessarily indicative of actual values or predictive of future results or values, which may be significantly more or less favourable than as set forth below. In addition, analyses relating to the value of Nightstar do not purport to be appraisals or reflect the prices at which Nightstar may actually be sold. Accordingly, the assumptions and estimates used in, and the results derived from, the financial analyses are inherently subject to substantial uncertainty. Except as otherwise noted, the following quantitative information, to the extent that it is based on market data, is based on market data as it existed on or before 1 March 2019 (the last trading day before the public announcement of the Acquisition) and is not necessarily indicative of current market conditions.

Selected Comparable Company Analysis

Centerview reviewed certain financial information of Nightstar and compared it to corresponding financial information of certain publicly traded, gene therapy-focused companies with market capitalizations of less than approximately USD 1.5 billion and clinical stage assets that Centerview selected based on its experience and professional judgment (which are referred to as the “selected companies” in this summary of Centerview’s opinion). Although none of the selected companies is directly comparable to Nightstar, the companies listed below were chosen by Centerview, among other reasons, because they are publicly traded, gene therapy-focused companies with clinical stage assets with certain operational, business and/or financial characteristics that, for purposes of Centerview’s analysis, may be considered similar to those of Nightstar.

However, because none of the selected companies is exactly the same as Nightstar, Centerview believed that it was inappropriate to, and therefore did not, rely solely on the quantitative results of the selected public company analysis. Accordingly, Centerview also made qualitative judgments, based on its experience and professional judgment, concerning differences between the operational, business and/or financial characteristics of Nightstar and the selected companies that could affect the public trading values of each in order to provide a context in which to consider the results of the quantitative analysis.

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The selected companies are set forth below:

Centerview calculated and compared forward revenue multiples for the selected companies based on information it obtained from public filings and other data sources. With respect to each of the selected companies, Centerview calculated enterprise value (calculated as the market value of common equity (determined using the treasury stock method and taking into account outstanding in-the-money options, warrants, restricted stock units and performance share units), plus certain liabilities less cash and cash equivalents) as a multiple of consensus estimated risk-adjusted revenues for calendar year 2024, which is referred to in the table above as the “EV/2024E Revenue Multiple.”

Based on this analysis and other considerations that Centerview deemed relevant in its professional judgment and expertise, Centerview applied a multiple range, derived from the estimated 2024 revenue multiples of the selected companies, of 0.5x to 1.0x to Nightstar’s estimated risk-adjusted calendar year 2024 revenue for each of the Case A Forecasts of USD 1.080 billion, Case B Forecasts of USD 633 million and Case C Forecasts of USD 415 million as set forth in the section entitled “Certain Forecasts” in paragraph 6 of Part I (Letter from the Chairman of Nightstar Therapeutics plc) and added to each Nightstar’s estimated net cash position as of 28 February 2019 of USD 158 million, and divided each result by the number of fully-diluted Nightstar Shares outstanding (determined using the treasury stock method and taking into account outstanding options and restricted stock units) as of 28 February 2019 based on the Internal Data provided by Nightstar management.

This analysis resulted in the following implied per share equity value ranges for the Nightstar Shares:

Centerview compared these ranges to the USD 25.50 per Nightstar Share proposed to be paid to the holders of the Nightstar Shares (other than Excluded Shares) pursuant to the Implementation Agreement.

Discounted Cash Flow Analysis

Centerview performed a discounted cash flow analysis of Nightstar based on Case A Forecasts, Case B Forecasts and Case C Forecasts as set forth in the section entitled “Certain Forecasts” in paragraph 6 of

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Part I (Letter from the Chairman of Nightstar Therapeutics plc). A discounted cash flow analysis is a traditional valuation methodology used to derive a valuation of an asset or set of assets by calculating the “present value” of estimated future cash flows of the asset or set of assets. “Present value” refers to the current value of future cash flows and is obtained by discounting those future cash flows by a discount rate that takes into account macroeconomic assumptions and estimates of risk, the opportunity cost of capital, expected returns and other appropriate factors.

In performing this analysis, Centerview calculated a range of equity values for Nightstar by (i) discounting to present value as of 31 March 2019 using discount rates ranging from 12.5% to 15.0% (reflecting Centerview’s analysis of Nightstar’s weighted average cost of capital, which is based on the assumption that Nightstar continues to hold no debt) and the mid-year convention: (a) the forecasted risk adjusted, after-tax unlevered free cash flows of Nightstar based on each of Case A Forecasts, Case B Forecasts and Case C Forecasts over the period beginning on 1 April 2019 and ending on 31 December 2035 utilised by Centerview as set forth in the section entitled “Certain Forecasts” in paragraph 6 of Part I (Letter from the Chairman of Nightstar Therapeutics plc) and (b) the forecasted tax savings from usage of net operating losses and future losses and (ii) adding to the foregoing results (i) Nightstar’s net cash balance as of 28 February 2019 of USD 158 million, as set forth in the Internal Data and (ii) an assumed USD 75 million equity financing in 2019, issued at USD 15 per share with USD 14 per share proceeds, to fund future development and commercialisation of Nightstar’s products. Centerview divided the result of the foregoing calculations by the number of fully-diluted outstanding Nightstar Shares (determined using the treasury stock method and taking into account outstanding options and restricted stock units) as at 28 February 2019 based on the Internal Data provided by Nightstar management and adjusting for the assumed USD 75 million equity financing in 2019.

This analysis resulted in the following implied per share equity value ranges for the Nightstar Shares:

Centerview compared these ranges to the USD 25.50 per Nightstar Share proposed to be paid to the holders of the Nightstar Shares (other than Excluded Shares) pursuant to the Implementation Agreement.

Other Factors

Centerview noted for the Nightstar Board certain additional factors solely for informational purposes, including, among other things, the following:

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General

The preparation of a financial opinion is a complex analytical process involving various determinations as to the most appropriate and relevant methods of financial analysis and the application of those methods to the particular circumstances and, therefore, a financial opinion is not readily susceptible to summary description. In arriving at its opinion, Centerview did not draw, in isolation, conclusions from or with regard to any factor or analysis that it considered. Rather, Centerview made its determination as to fairness on the basis of its experience and professional judgment after considering the results of all of the analyses.

Centerview’s financial analyses and opinion were only one of many factors taken into consideration by the Nightstar Board in its evaluation of the Acquisition. Consequently, the analyses described above should not be viewed as determinative of the views of the Nightstar Board or management of Nightstar with respect to the Consideration or as to whether the Nightstar Board would have been willing to determine that a different consideration was fair. The consideration for the transaction was determined through arm’s-length negotiations between Nightstar and Bidder and was approved by the Nightstar Board. Centerview provided advice to Nightstar during these negotiations. Centerview did not, however recommend any specific amount of consideration to Nightstar or the Nightstar Board or that any specific amount of consideration constituted the only appropriate consideration for the Acquisition.

Centerview is a securities firm engaged directly and through affiliates and related persons in a number of investment banking, financial advisory and merchant banking activities. In the two years prior to the date of its written opinion, except for Centerview’s current engagement with Nightstar, Centerview has not been engaged to provide financial advisory or other services to Nightstar, and Centerview has not received compensation from Nightstar during such period. In the two years prior to the date of its written opinion, Centerview has not been engaged to provide financial advisory or other services to Syncona Portfolio Limited, Biogen, Bidder or Bidco, and Centerview has not received compensation from Syncona Portfolio Limited, Biogen, Bidder or Bidco during such period. Centerview may provide financial advisory and other services to or with respect to Nightstar or Bidder or their respective affiliates in the future, for which Centerview may receive compensation. Certain (i) of Centerview’s and its affiliates’ directors, officers, members and employees, or family members of such persons, (ii) of Centerview’s affiliates or related investment funds and (iii) investment funds or other persons in which any of the foregoing may have

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financial interests or with which they may co-invest, may at any time acquire, hold, sell or trade, in debt, equity and other securities or financial instruments (including derivatives, bank loans or other obligations) of, or investments in, Nightstar, Bidder, or any of their respective affiliates, or any other party that may be involved in the Acquisition.

The Nightstar Board selected Centerview as its financial advisor in connection with the Acquisition based on Centerview’s reputation and experience. Centerview is an internationally recognised investment banking firm that has substantial experience in transactions similar to the Acquisition.

In connection with Centerview’s services as the financial advisor to the Nightstar Board, Nightstar has agreed to pay Centerview an aggregate fee of USD 15,792,939, USD 2,000,000 of which was payable upon the rendering of Centerview’s opinion and USD 13,792,939 of which is payable contingent upon consummation of the Acquisition. In addition, Nightstar has agreed to reimburse certain of Centerview’s expenses arising, and to indemnify Centerview against certain liabilities that may arise, out of Centerview’s engagement.

Nightstar, which does not yet have any marketed products, does not make public long-term projections as to future revenues, earnings or other results due to the uncertainty of the underlying assumptions and estimates. However, in connection with the Nightstar Board’s evaluation of the Acquisition and other strategic alternatives available to the company, Nightstar’s management prepared certain long-range, risk-adjusted financial projections for the years 2019 through 2035 for three cases reflecting varying assumptions. The Forecasts were provided to the Nightstar Board and to Centerview, but not to Bidder.

The Forecasts were based on certain internal assumptions about the probability of success and regulatory approval, launch timing, pricing, market growth, market share, competition and other relevant factors relating to the commercialisation of Nightstar’s product candidates. Nightstar’s management considered it appropriate to prepare forecasts presenting three scenarios to reflect varying commercial assumptions and probability of success with respect to Nightstar’s lead clinical product candidates. All of these factors are difficult to predict and many are beyond Nightstar’s control. Accordingly, there can be no assurance that the Forecasts will be realised, and actual results may vary materially from those shown.

Nightstar is electing to summarise the Forecasts in this document to provide Nightstar Shareholders access to certain prospective financial information that was made available to the Nightstar Board at the time of its evaluation of the Acquisition and the Implementation Agreement, and to Centerview. The Forecasts are not included to influence a Nightstar Shareholder’s decision whether to vote in favour of the Acquisition. The Forecasts were not prepared with a view toward public disclosure, nor were they prepared with a view toward compliance with the published guidelines of the SEC, the guidelines established by the American Institute of Certified Public Accountants for preparation and presentation of financial forecasts, or U.S. generally accepted accounting principles. In addition, the Forecasts were not prepared with the assistance of, or reviewed, compiled or examined by, independent accountants. Nightstar makes and has made no representation to Bidder or Bidco, in the Implementation Agreement or otherwise, concerning the Forecasts.

The Forecasts are summarised below:

Case A Forecasts

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Case B Forecasts

Case C Forecasts

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The inclusion of the above financial projections in this document should not be regarded as an indication that any of Nightstar or its affiliates, advisors or representatives considered, or that Nightstar, Bidder, Bidco or their respective affiliates, advisors or representatives consider such projections to be predictive of actual future events, and they should not be relied upon as such. None of Nightstar, Bidder, Bidco or their respective affiliates, advisors, officers, directors or representatives can give any assurance that actual results will not differ from the projections and none of them undertakes any obligation to update or otherwise revise or reconcile such projections to reflect circumstances existing after the date such projections were generated or to reflect the occurrence of future events, even in the event that any or all of the assumptions underlying the projections are shown to be in error. None of Nightstar, Bidder or Bidco intends to make publicly available any update or other revisions to the projections described above, except as required by law.

All financial projections are forward-looking statements. These and other forward-looking statements are expressly qualified in their entirety by the risks and uncertainties identified above and the cautionary statements contained in Nightstar’s Form 10-K for the year ended 31 December 2018. Any provisions of the Private Securities Litigation Reform Act of 1995 that may be referenced in this document or in Nightstar’s other filings with the SEC are not applicable to any forward-looking statements made in connection with the Acquisition. Please refer to the discussion entitled “Warning Regarding Forward-Looking Statements.”

In light of the foregoing factors and the uncertainties inherent in these projections, shareholders are cautioned not to place undue, if any, reliance on these projections.

Bidder has received irrevocable undertakings to vote or procure votes in favour of the resolutions relating to the Scheme at the Nightstar Shareholder Meetings (or in the event that the Acquisition is implemented by way of an Offer, to accept or procure acceptance of such Offer) from the Nightstar Directors, in respect of their entire beneficial holdings, amounting to 758,019 Nightstar Shares, in aggregate, representing approximately 2.3 per cent. of the issued share capital of Nightstar as at 5 April 2019, being the last practicable date prior to the publication of this document.

Bidco has also received irrevocable undertakings from Syncona Portfolio Limited and New Enterprise Associates, both Nightstar Shareholders, to vote or procure votes in favour of the resolutions relating to the Scheme at the Nightstar Shareholder Meetings (or in the event that the Acquisition is implemented by way of an Offer, to accept or procure acceptance of such Offer) in respect of their entire beneficial holdings, amounting to 19,868,280 Nightstar Shares, in aggregate, representing approximately 59.2 per cent. of the issued share capital of Nightstar as at 5 April 2019, being the last practicable date prior to the publication of this document.

The Irrevocable Undertakings given by the Nightstar Directors will cease to be binding if:

and will remain binding if a higher competing offer is made for Nightstar.

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The Irrevocable Undertakings given by Syncona Portfolio Limited and New Enterprise Associates will cease to be binding if:

and will remain binding if a higher competing offer is made for Nightstar.

The form of the Director’s Irrevocable Undertaking is attached hereto as Annex C and the form of the Shareholder’s Irrevocable Undertaking is attached hereto as Annex D. Further details of these Irrevocable Undertakings are set out in Part V (Additional Information) of this document.

The Nightstar Board, together with Nightstar’s senior management team, regularly reviews Nightstar’s performance, growth prospects and overall strategic direction and evaluates potential opportunities to strengthen Nightstar’s businesses and enhance shareholder value. From time to time these reviews and evaluations have included Nightstar’s strategy as a stand-alone company and potential opportunities for business combinations, strategic partnerships, mergers and acquisitions and other financial and strategic alternatives.

From time to time prior to Nightstar’s initial public offering in September 2017, representatives of Nightstar engaged in preliminary discussions with representatives of Biogen, Company A and Company B regarding potential strategic partnerships with Nightstar. These conversations were primarily focused on the licensing of Nightstar’s product candidate for choroideremia, NSR-REP1.

On 28 June 2018, a representative of Company A emailed David Fellows, Nightstar’s Chief Executive Officer and a member of the Nightstar Board, expressing Company A’s interest in Nightstar’s product candidate for choroideremia and desire to conduct diligence under a non-disclosure agreement. During a subsequent telephone call on 5 July 2018, a representative of Company A indicated to Mr. Fellows that Company A might be interested in a portfolio of Nightstar’s product candidates, including NSR-RPGR, Nightstar’s product candidate for the treatment of XLRP, and requested an in-person or telephonic meeting in August to conduct clinical as well as chemistry, manufacturing and controls (“CMC”) diligence.

On 10 July 2018, Nightstar sent a draft non-disclosure agreement to Company A. The parties negotiated the form over the following days, and the non-disclosure agreement was subsequently executed on 8 August 2018. Neither this non-disclosure agreement, nor the non-disclosure agreements Nightstar entered into with Company B and Company C, contained a stand-still provision.

On 30 July 2018, Senthil Sundaram, Nightstar’s Chief Financial Officer, had a discussion with Daniel Karp, Biogen’s Executive Vice President, Corporate Development, regarding Nightstar and Biogen’s interests in the field of gene therapy.

On 16 August 2018, Mr. Sundaram had a call with a representative of Company A to discuss next steps in its evaluation of a potential transaction. Company A indicated a specific interest in Nightstar’s three most advanced product candidates (NSR-REP1, NSR-RPGR and NSR-ABCA4) and its manufacturing capabilities, and requested a meeting in September with Nightstar management to review the product candidates in greater detail.

On 24 August 2018, David Mott, a non-executive director of Nightstar, had a meeting with a representative of Company B to discuss general strategic topics in biotechnology. During this meeting, the representative of Company B expressed an interest in Nightstar’s portfolio of product candidates and requested a meeting with Mr. Fellows to discuss that interest in more detail.

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On 20 September 2018, Mr. Fellows met with a representative of Company B to discuss Company B’s strategic interests in the field of ophthalmology gene therapy.

On 22 September 2018, Nightstar publicly announced preliminary safety and efficacy results from the Phase 1 dose escalation study in the XIRIUS trial for NSR-RPGR, which provided the basis for an early clinical proof of concept to progress to the Phase 2/3 expansion study in the XIRIUS trial that is currently in progress.

On 2 October 2018, Nightstar completed a follow-on equity offering resulting in net proceeds of approximately USD 77.1 million.

On 4 October 2018, Mr. Sundaram met with representatives of Company C at an industry conference, where they discussed Company C’s strategic interests in ophthalmology gene therapy and desire to learn more about Nightstar’s product candidates in a meeting with Nightstar’s management.

Throughout September, October, November and December of 2018, representatives of Nightstar, including members of Nightstar management, held in-person and telephonic meetings with representatives of Company A, Company B and Company C as they conducted due diligence on Nightstar’s product candidates and operations and to discuss the ongoing status of each counterparty’s interest in a potential transaction with Nightstar.

On 24 October 2018, at a special meeting of the Nightstar Board convened by telephone, representatives of Nightstar’s financial advisor, Centerview, discussed with the Nightstar Board market perspectives on Nightstar and a general overview of process considerations relating to third-party expressions of interest in a strategic transaction with Nightstar.

On 4 November 2018, Mr. Fellows received an email from a representative of Company B regarding its interest in engaging in confidential discussions regarding a strategic transaction with Nightstar. After discussion with Bryan Yoon, Nightstar’s General Counsel, Mr. Fellows confirmed with Company B the existence of an effective confidentiality agreement from the interactions between the two companies prior to Nightstar’s IPO.

On 6 November 2018, Mr. Sundaram had a telephone call with Mr. Karp, on which Mr. Karp confirmed that Biogen was open to continuing discussions with Nightstar.

On 7 November 2018, following initial discussions and due diligence by representatives of Company A, Nightstar provided Company A with access to a limited electronic data room to conduct additional diligence.

On 13 November 2018, Nightstar announced the planned initiation of a Phase 2/3 expansion study in the XIRIUS trial of NSR-RPGR in XLRP.

On 16 November 2018, Mr. Fellows met with a representative from Company A to discuss the key assumptions that Company A was using to develop its valuation forecast of Nightstar’s product candidates. Company A confirmed that it was preparing a non-binding offer and inquired regarding Nightstar’s valuation expectations. Mr. Fellows responded that any offer should reflect the fundamental future value of Nightstar’s complete pipeline of product candidates.

On 30 November 2018, representatives of Nightstar’s management met with representatives of Company B at their offices and conducted an in-depth review of Nightstar’s infrastructure and pipeline.

On 5 December 2018, representatives of Company A emailed Mr. Fellows to note that Company A was in the process of finalising its valuation assumptions, and that its proposal would be submitted within the following two weeks.

On 10 December 2018, Mr. Fellows and Mr. Sundaram had a call with representatives of Company B to discuss process and timing considerations. Company B indicated on the call that it would send an indication of interest to Nightstar later in that week and expressed Company B’s desire to move quickly on diligence.

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On 12 December 2018, Nightstar received a non-binding proposal from Company B outlining the terms of an acquisition of Nightstar at a price of USD 20.50 per share in cash (representing a premium of 70.1 per cent. over the Closing Price on 12 December 2018), subject to satisfactory completion of due diligence and negotiation of definitive documentation.

On 14 December 2018, the Nightstar Board held a telephonic meeting to review the non-binding proposal received from Company B on 12 December 2018, which was attended by Mr. Fellows, Mr. Sundaram and Mr. Yoon as well as representatives of Skadden and Centerview. The Nightstar Board instructed Nightstar management to provide Company B with access to a limited electronic data room to conduct additional diligence. At the same meeting, the Nightstar Board authorised Nightstar’s management and Centerview to contact Company A, Company C and Biogen to better understand their desire to participate in a targeted evaluation process for a potential acquisition of Nightstar.

On 18 December 2018, a representative of Centerview, at the direction of the Nightstar Board, invited Biogen to participate in Nightstar’s evaluation process with respect to a possible acquisition and Nightstar sent Biogen a draft of a non-disclosure agreement with Nightstar. Subsequently, on 21 December 2018, Biogen and Nightstar entered into a non-disclosure agreement and representatives of Nightstar held a telephonic meeting with representatives of Biogen on which the participants reviewed a confidential management presentation that related to Nightstar’s lead product candidates and CMC capabilities. On the same day, Biogen was granted access to a limited electronic data room.

On 19 December 2018, a representative of Centerview had a telephone call with a representative of Company A, who indicated that Company A was committed to submitting a non-binding indication of interest during the week of 14 January 2019.

On 20 December 2018, at the direction of the Nightstar Board, a representative of Centerview sent a draft non-disclosure agreement to Company C. Company C and Nightstar negotiated the form over the following days, and on 2 January 2019, Company C and Nightstar executed the non-disclosure agreement.

On 21 December 2018, Mr. Fellows received a telephone call from the chief executive officer of Company B, who indicated that Company B had a high degree of interest in acquiring Nightstar and wanted to move quickly to complete a transaction pending access to more detailed information in an electronic data room. On the same day following this call, Company B was provided with access to a limited electronic data room.

Throughout January of 2019, Nightstar continued to hold in-person and telephonic meetings with each of Biogen, Company A, Company B and Company C to address outstanding diligence topics.

On 3 January 2019, Company C was provided with access to a limited electronic data room.

On 7 January 2019, Mr. Fellows and Mr. Sundaram met with representatives of Company A during an industry conference. During that meeting the parties discussed the transaction process, key outstanding areas of Company A’s due diligence and Company A’s goal to provide a written indication of interest during the week of 21 January 2019.

Also on 7 January 2019, Mr. Fellows and Mr. Sundaram met with representatives of Company B, and Mr. Mott separately met with another representative of Company B. These conversations centered on key outstanding areas of Company B’s due diligence.

On 8 January 2019, Mr. Fellows and Mr. Sundaram met with Mr. Karp, Nicholas Scarchilli, Biogen’s Director, Business Development and other Biogen employees during an industry conference to discuss Biogen’s continued interest in a strategic transaction with Nightstar.

On 15 January 2019, representatives of Nightstar met with representatives of Biogen at Biogen’s offices to address its key outstanding areas of due diligence. The following day, on 16 January 2019, representatives of Nightstar and Biogen held a telephone call to conduct CMC diligence.

On 16 January 2019, representatives of Nightstar met with the FDA to review non-clinical, clinical and CMC questions relating to the NSR-RPGR development program.

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On 18 January 2019, after representatives of Centerview disclosed to the Nightstar Board certain relationships that Centerview had with potential parties that were likely to be contacted in connection with a potential transaction, representatives of Skadden discussed with the Nightstar Board its duties with respect to potential conflicts of interests. Following this discussion, the Nightstar Board approved the engagement of Centerview as financial adviser in connection with a potential transaction.

During the week of 14 January 2019, at the direction of the Nightstar Board, representatives of Centerview directed each of Biogen, Company A, Company B and Company C to submit indications of interest during the week of 21 January 2019.

On 22 January 2019, Nightstar entered into an engagement letter with Centerview pursuant to which Centerview was formally retained as financial advisor to Nightstar in connection with a potential transaction.

On 23 January 2019, Mr. Scarchilli informed representatives of Centerview that Biogen needed additional time to complete diligence and finalise its proposal. Subsequently, on 25 January 2019, Mr. Sundaram and Mr. Scarchilli had a telephone call on which Mr. Scarchilli indicated that Biogen expected to submit a formal bid during the week of 30 January 2019.

Also on 23 January 2019, a representative of Company C informed representatives of Centerview that Company C would need additional time to prepare and submit a proposal.

On 29 January 2019, Company A informed representatives of Centerview that it was determining next steps internally and that if Company A elected to move forward, it would submit a bid letter by the end of that week.

Also on 29 January 2019, Mr. Fellows, Mr. Sundaram and Dr. Tuyen Ong, Nightstar’s Chief Development Officer, met with representatives of Company B at their offices. Later that day, the same members of Nightstar’s management met representatives of Company C at their offices. The participants from Company B and Company C, at their respective meetings, conducted an in-depth review of Nightstar’s clinical and preclinical programs.

On 30 January 2019, Nightstar received a non-binding proposal from Biogen outlining the terms of an acquisition of Nightstar at a price of USD 19.00-21.00 per share in cash (representing a premium of 47.2-62.7 per cent. over the Closing Price on 30 January 2019), subject to satisfactory completion of due diligence and obtaining requisite approvals.

On 1 February 2019, at the direction of the Nightstar Board, representatives of Centerview discussed Biogen’s proposal with Mr. Scarchilli, stating that the Nightstar Board would likely not agree to a sale in the price range proposed on 30 January.

On 2 February 2019, Mr. Fellows, Mr. Sundaram and Mr. Yoon updated Chris Hollowood, Nightstar’s non-executive Chairman, and Mr. Mott regarding the developments with each of the parties, during which it was agreed that Nightstar would grant expanded data room access to Biogen but that Biogen’s 30 January offer price of USD 19.00-21.00 per share was well below the stand-alone value of the business. Afterwards, Mr. Fellows provided telephonic updates to the other members of Nightstar’s Board.

On 4 February 2019, representatives of Centerview held a telephone call with representatives of Biogen and Biogen’s financial advisor, Goldman Sachs, on which representatives of Centerview relayed the conclusions of the 2 February 2019 discussion of Nightstar Directors and noted that if Nightstar did not receive an attractive proposal, the Nightstar Board considered remaining a stand-alone company to be a promising alternative. Later in the day on 4 February, Biogen was granted access to a second electronic data room containing a comprehensive set of confidential materials.

Also on 4 February 2019, Company A communicated to representatives of Centerview that it did not wish to pursue an acquisition transaction with Nightstar at that time, but it expressed an interest in potentially pursuing a strategic partnership involving a collaboration and license arrangement with Nightstar.

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Also on 4 February 2019, Company C communicated to representatives of Centerview that it did not desire to pursue an acquisition transaction at such time.

On 5 February 2019, Mr. Fellows had a telephone call with a representative of Company A to further discuss Company A’s interest in pursuing a strategic partnership.

Also on 5 February 2019, Company B communicated to representatives of Nightstar that it did not wish to pursue an acquisition transaction with Nightstar at that time, but it also expressed an interest in possibly pursuing a strategic partnership.

On 7 February 2019, at the direction of the Nightstar Board, representatives of Centerview delivered a bid process letter to Biogen.

Also on 8 February 2019, Mr. Fellows spoke to a representative of Company B requesting details around a potential strategic partnership between the parties.

On 9 February 2019, a representative of Company A and Mr. Fellows discussed potential terms for a possible strategic collaboration. Mr. Fellows responded that he would discuss the proposal with the Nightstar Board.

Throughout February 2019, Biogen continued to perform due diligence on Nightstar, including reviewing materials made available in Nightstar’s electronic data room and holding telephonic and in-person meetings with members of Nightstar’s management and certain Nightstar employees.

On 11 February 2019, representatives of Biogen provided an extensive diligence request list to representatives of Nightstar. That list was subsequently updated by Biogen throughout the month of February.

On 12 February 2019, representatives of Nightstar’s legal M&A counsel, Skadden, sent an initial draft Implementation Agreement to representatives of Biogen’s legal counsel, Ropes, along with drafts of the Announcement and the Irrevocable Undertakings.

On 18 and 19 February 2019, a representative of Company A and Mr. Fellows discussed a revised set of terms for a potential strategic collaboration.

Between 20 February and 3 March 2019, representatives of Nightstar and representatives of Company B exchanged communications seeking to set a meeting to discuss a potential strategic collaboration.

On 21 February 2019, representatives of Centerview and Goldman Sachs discussed the process and valuation for a potential transaction. Representatives of Centerview indicated, at the direction of the Nightstar Board, that Nightstar remained focused on announcing a transaction on 4 March 2019 and that the Nightstar Board would not find a modest increase to Biogen’s prior proposal acceptable.

On 22 February 2019, a representative of Company A emailed Mr. Fellows requesting on update on Nightstar’s interest in a potential collaboration. Mr. Fellows replied with a collaboration framework, which the representative of Company A responded with a number of clarifying questions.

On 25 February 2019, representatives of Ropes sent a revised draft Implementation Agreement to representatives of Skadden. Following delivery of the draft Implementation Agreement, representatives of Centerview held a call with representatives of Biogen and Goldman Sachs to discuss process. Representatives of Centerview stated that its prior discussions regarding guidance on timing and price had not changed.

On 26 February 2019, the Nightstar Board held a regularly scheduled meeting. Following adjournment of the meeting, the Nightstar Board convened a special meeting that was attended by Mr. Fellows, Mr. Sundaram and Mr. Yoon as well as representatives of Skadden and Centerview. During the special meeting, the Nightstar Board reviewed certain financial projections that had been prepared by Nightstar’s management and discussed the valuation implied by such projections. Additionally, representatives of Skadden reviewed with the Nightstar Board the revised terms of the Implementation Agreement as proposed

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by Ropes, and representatives of Centerview reviewed with the Nightstar Board strategic considerations and next steps in responding to the Implementation Agreement markup. Also at this meeting, the Nightstar Board received an update from Nightstar’s management regarding the status of discussions with Company A with respect to a possible strategic partnership between the parties. Later on the same day, representatives of Centerview held a call with representatives of Goldman Sachs to discuss process. Representatives of Centerview stated that its prior discussions regarding timing and price had not changed and recent transaction premiums supported a higher price.

Also on 26 February 2019, representatives of Skadden sent a draft of the Disclosure Schedules to the Implementation Agreement to representatives of Ropes. The parties negotiated and revised the Disclosure Schedules during the period between this initial delivery and finalisation of the Implementation Agreement on 3 March 2019.

On 27 February 2019, representatives of Nightstar, Skadden, Biogen and Ropes spoke by telephone regarding open issues in the Implementation Agreement. On that same day, Skadden sent a revised draft of the Implementation Agreement to Ropes.

On 1 March 2019, Nightstar received a revised proposal from Biogen that had been approved by Biogen’s Board outlining the terms of an acquisition of Nightstar at a price of USD 25.00 per share in cash (representing a premium of 64.9 per cent. to the Closing Price on 1 March 2019). Immediately after Biogen’s revised proposal was delivered, Ropes sent a revised draft of the Implementation Agreement to Skadden.

Thereafter on 1 March 2019, the Nightstar Board held a special meeting by telephone that was attended by Mr. Fellows, Mr. Sundaram and Mr. Yoon as well as representatives of Skadden and Centerview. The Nightstar Board reviewed the terms of Biogen’s revised proposal, and representatives of Skadden reviewed with the Nightstar Board the revised terms of the Implementation Agreement as proposed by Ropes. Following this meeting, representatives of Centerview, at the direction of the Nightstar Board, had a call with representatives of Goldman Sachs on which representatives of Centerview conveyed that the price proposed in Biogen’s proposal of 1 March 2019 was not sufficient for the Nightstar Board to agree to a transaction and that remaining independent or entering into a strategic collaboration agreement remained viable options, so Biogen would need to increase its proposal to reach an agreement. At the direction of the Nightstar Board, representatives of Centerview also contacted Mr. Karp and communicated that Biogen’s proposal was inadequate and a price of USD 26.50 per share would be compelling to the Nightstar Board.

Also on 1 March 2019, a representative of Company A requested a call with Mr. Fellows to discuss further potential collaboration terms. A representative of Company A also contacted Dr. Hollowood to inquire further about the Board’s interest in a strategic partnership. Dr. Hollowood responded by email the same day.

Throughout the period of 1 March to 3 March 2019, representatives of Ropes and representatives of Skadden discussed and exchanged comments to the draft Implementation Agreement and related transaction documents.

The Nightstar Board held a special meeting on 2 March 2019 by telephone that was attended by Mr. Fellows, Mr. Sundaram and Mr. Yoon as well as representatives of Skadden and Centerview. During that meeting Skadden delivered an update to the Nightstar Board regarding the status of negotiations on the Implementation Agreement and Disclosure Schedules thereto.

Also on 2 March 2019, Nightstar received a revised proposal from Biogen on substantially identical terms to the proposal of 1 March 2019 at a price of USD 25.50 per share in cash.

Also on 2 March 2019, Mr. Fellows contacted a representative of Company A to progress discussions around a potential strategic collaboration during the week of 4 March 2019.

Also on 3 March 2019, following further negotiations between representatives of Skadden and representatives of Ropes, the parties agreed on the final form of Implementation Agreement. In addition, the parties finalised the Announcement and Disclosure Schedules to the Implementation Agreement.

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Also on 3 March 2019, the Nightstar Board held a special meeting to consider Biogen’s revised proposal that was attended by representatives of Skadden, and, for portions of the meeting, Mr. Fellows, Mr. Sundaram and Mr. Yoon and representatives of Centerview. Representatives of Skadden reviewed with the Nightstar Board their fiduciary duties applicable to a sale of Nightstar and described certain key legal terms of the Implementation Agreement. Representatives of Centerview reviewed with the Nightstar Board Centerview’s financial analysis of the proposed consideration of USD 25.50 per share in cash, and rendered an oral opinion to the Nightstar Board, which was subsequently confirmed by delivery of a written opinion dated such date, that, as of such date and based upon and subject to various assumptions made, procedures followed, matters considered and qualifications and limitations upon the review undertaken in preparing its opinion, the USD 25.50 per share in cash to be paid to the holders of the Nightstar Shares (other than as specified in such opinion) pursuant to the Implementation Agreement was fair, from a financial point of view, to such holders. For a detailed discussion of Centerview’s opinion, please see the section entitled “Opinion of Nightstar’s Financial Advisor” in paragraph 5 of Section I (Letter from the Chairman of Nightstar Therapeutics plc). The written opinion delivered by Centerview is attached to this document as Annex B.

Following additional discussion and consideration of the Implementation Agreement and the Acquisition, including a discussion of the comparative value that Nightstar Shareholders might realise from Nightstar continuing as a stand-alone business, the Nightstar Board unanimously (i) resolved that the terms of the Acquisition, including the terms of the Implementation Agreement are fair and reasonable and that proceeding with the Acquisition is likely to promote the success of Nightstar for the benefit of its shareholders, having had regard to the interests of other stakeholders, and (ii) resolved to recommend that Nightstar’s shareholders vote in favour of the Scheme at the Court Meeting and the Resolutions to be proposed at the General Meeting.

Later in the day on 3 March 2019, representatives of Nightstar telephoned representatives of Biogen to inform Biogen that the Nightstar Board had unanimously approved the proposed transaction with Biogen. Thereafter on 4 March 2019, Nightstar, Bidder and Bidco executed the Implementation Agreement.

Before the opening of trading on Nasdaq on 4 March 2019, Nightstar and Biogen issued the Announcement to announce the execution of the Implementation Agreement.

On 9 April 2019, Nightstar filed this document.

In evaluating the Implementation Agreement, the Acquisition and the other transactions contemplated by the Implementation Agreement, the Nightstar Board consulted with the senior management of Nightstar, and representatives of Centerview and Skadden. In the course of making the determination that the Implementation Agreement, the Acquisition and the other transactions contemplated by the Implementation Agreement are fair and reasonable and are likely to promote the success of Nightstar for the benefit of its shareholders, having had regard to the interests of other stakeholders, and to recommend that Nightstar’s shareholders vote in favour of the Scheme at the Court Meeting and the Resolutions to be proposed at the General Meeting, the Nightstar Board considered numerous reasons, including the following material reasons and benefits of the Acquisition:

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In reaching its determinations and recommendations described above, the Nightstar Board also considered the following potentially negative reasons:

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The foregoing discussion of the reasons considered by the Nightstar Board is not intended to be exhaustive, but rather includes the principal reasons considered by the Nightstar Board. The Nightstar Board collectively reached the conclusion to approve the Implementation Agreement, the Acquisition and the other transactions contemplated by the Implementation Agreement in light of the various reasons described above and other reasons that the members of the Nightstar Board believed were appropriate. The Nightstar Board did not assign relative weights to the foregoing reasons or otherwise determine that any one reason was of greater or lesser importance. Rather, the Nightstar Board viewed its positions and recommendation as being based on the totality of information presented to, and considered by, the Nightstar Board. In considering the reasons discussed above, individual directors may have given different weights to different reasons.

On the Effective Date, all outstanding stock options, restricted shares and restricted stock units awarded pursuant to the 2017 Equity Incentive Plan will be cancelled and cease to be outstanding, and the holder will be entitled to receive a payment equal to the Consideration with respect to each Nightstar Share subject to the Award less any applicable exercise or purchase price and applicable withholdings.

Following the Effective Date, any Nightstar Shares subject to or issued in connection with the vesting or exercise of any Pre-IPO Equity Award will be immediately acquired by Bidco from the relevant Award holder in exchange for the Consideration for each Nightstar Share so acquired.

Awards held by the Nightstar Directors, which consist of stock options and restricted shares, will be treated in the same way as the Awards held by other participants in the Nightstar Share Schemes.

Any Consideration payable to Award holders in the Nightstar Share Schemes will be paid to them in accordance with the terms of the Scheme, subject to any arrangements that are required to be put in place to effect the payment of any income tax and/or social security contributions that arise in connection with the Awards.

Holders of Deferred Shares will not receive the Consideration. Subject to the passing of the Re-designation Special Resolution and the Ordinary Resolution, Nightstar intends to repurchase all Deferred Shares ahead of the Effective Date under the terms of the Buy-Back Agreements.

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Award holders under the Nightstar Share Schemes will be written to separately to inform them of the effect of the Scheme on their rights under the Nightstar Share Schemes, including any appropriate proposals being made to them about their rights. Further details are set out in paragraph 9 of Part II (Explanatory Statement) of this document.

It is intended that the Acquisition will be implemented by way of a Court-sanctioned scheme of arrangement between Nightstar and the Nightstar Scheme Shareholders, made under Part 26 of the Companies Act 2006. The procedure involves, amongst other things, an application by Nightstar to the Court to sanction the Scheme, in consideration for which the Nightstar Scheme Shareholders will receive the Consideration. The purpose of the Scheme is to provide for Bidco to become the owner of the entire issued and to be issued share capital of Nightstar.

The Scheme will only become Effective if, amongst other things, the following events occur on or before the Long Stop Date:

The Court Meeting and the General Meeting are scheduled to be held on 8 May 2019.

Upon the Scheme becoming Effective: (i) it will be binding on all Nightstar Scheme Shareholders, irrespective of whether or not they attended or voted at the Court Meeting or the General Meeting (and if they attended and voted, whether or not they voted in favour); and (ii) share certificates in respect of Nightstar Shares will cease to be valid and entitlements to Nightstar Shares held within the CREST system will be cancelled. The Consideration will be despatched by Bidder to Nightstar Scheme Shareholders as soon as practicable after the Effective Date and in any event within 14 days of the Effective Date.

It is important that, for the Court Meeting, as many votes as possible are cast so that the Court may be satisfied that there is a fair representation of opinion of the Nightstar Scheme Shareholders. You are therefore urged to complete and return your Forms of Proxy, make an electronic appointment of a proxy or submit a proxy vote via CREST as soon as possible.

If the Scheme does not become Effective by the Long Stop Date, it will lapse and the Acquisition will not proceed (unless extended with the agreement of Bidder, Bidco, Nightstar and, if required, the approval of the Court).

The Scheme will be governed by English law and will be subject to the jurisdiction of the courts of England and Wales. The Scheme is not subject to U.S. law.

Further details of the Scheme and the Nightstar Shareholder Meetings are set out in paragraph 10 of Part II (Explanatory Statement) of this document.

Nightstar ADS Holders should refer to paragraph 15 of Part II (Explanatory Statement) of this document.

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Prior to the Scheme becoming Effective, a request will be made by Nightstar to Nasdaq to suspend trading of, and de-list, the Nightstar ADSs on Nasdaq, to take effect on, or shortly after, the Effective Date. Subsequently, the Nightstar ADSs and Nightstar Shares will be deregistered under the U.S. Exchange Act.

As soon as practicable after the Effective Date and after the cancellation of the listing and admission to trading of the Nightstar ADSs on Nasdaq, it is intended that Nightstar will be re-registered as a private limited company under the relevant provisions of the Companies Act 2006.

A summary of certain United Kingdom and United States taxation consequences of the implementation of the Scheme for certain Nightstar Shareholders is set out in paragraph 6 of Part V (Additional Information) of this document.

That summary does not constitute tax advice and does not purport to be a full analysis of all potential United Kingdom and United States tax consequences of the Acquisition. Nightstar Shareholders who are in any doubt about their taxation position, or who are subject to taxation in a jurisdiction outside of the United Kingdom and the United States are strongly advised to contact an appropriate independent professional adviser immediately.

Overseas Shareholders should refer to paragraph 17 of Part II (Explanatory Statement) of this document for further details.

Your attention is drawn to paragraph 19 of Part II (Explanatory Statement) of this document, which explains the actions you should take in relation to the Acquisition and the Scheme.

Your attention is drawn to Part II (Explanatory Statement) of this document (being the Explanatory Statement made in compliance with section 897 of the Companies Act 2006), which gives further details about the Acquisition and the terms of the Scheme that are set out in full at the end of this document. Please note that reading the information in Part II (Explanatory Statement) is not a substitute for reading the remainder of this document.

You are advised to read the whole of this document and not just to rely on the summary information in this letter.

Your attention is further drawn to the information contained in Part III (Conditions to and Further Terms of the Scheme and the Acquisition), Part V (Additional Information), Part VIII (The Scheme of Arrangement) and to the expected timetable of principal events set out on page xiv of this document.

For the reasons set out in this document, the Nightstar Directors, who have been so advised by Centerview as to the financial terms of the Acquisition, consider the terms of the Acquisition to be fair and reasonable. In providing advice to the Nightstar Directors, Centerview has taken into account the commercial assessments of the Nightstar Directors. Centerview’s advice to the Nightstar Directors is set out in its written opinion, dated 3 March 2019, attached hereto as Annex B and which also sets out in full the relevant assumptions and qualifications to such advice.

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Accordingly, the Nightstar Directors believe that the terms of the Acquisition, including the terms of the Implementation Agreement, are fair and reasonable and that proceeding with the Acquisition is likely to promote the success of Nightstar for the benefit of its shareholders, having had regard to the interests of other stakeholders, and unanimously recommend that Nightstar Scheme Shareholders vote in favour of the Scheme at the Court Meeting and the Resolutions to be proposed at the General Meeting, as they have irrevocably undertaken to do in respect of their own holdings of, in aggregate, 758,019 Nightstar Shares (representing approximately 2.3 per cent. of the issued ordinary share capital of Nightstar on 5 April 2019 (being the last practicable date prior to the publication of this document)).

Yours faithfully,

Chris Hollowood

Chairman

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PART II

EXPLANATORY STATEMENT

(in compliance with section 897 of the Companies Act 2006)

9 April 2019

RECOMMENDED ACQUISITION OF

NIGHTSTAR THERAPEUTICS PLC BY BIDCO

A WHOLLY-OWNED SUBSIDIARY OF BIDDER

On 4 March 2019, the Nightstar Board and Biogen announced that they had agreed to the terms of a recommended cash Acquisition of Nightstar by Bidco (a newly-incorporated company and wholly-owned subsidiary of Bidder) pursuant to which Bidco will acquire the entire issued and to be issued share capital of Nightstar. The Acquisition is to be effected by means of a scheme of arrangement under Part 26 of the Companies Act 2006, which requires the approval of Nightstar Shareholders and the sanction of the Court. The Acquisition remains subject to the terms and conditions set out in Part III (Conditions to and Further Terms of the Scheme and the Acquisition).

Your attention is drawn to the letter from the Chairman of Nightstar, Chris Hollowood, set out in Part I (Letter from the Chairman of Nightstar Therapeutics plc) of this document, which forms part of this Explanatory Statement. That letter contains, amongst other things, information on the background to and reasons for the recommendation and the unanimous recommendation by the Nightstar Board to Nightstar Shareholders to vote in favour of the resolutions to approve and implement the Acquisition to be proposed at the Nightstar Shareholder Meetings.

Your attention is also drawn to the information contained in Part III (Conditions to and Further Terms of the Scheme and the Acquisition) and Part V (Additional Information) of this document. The Scheme is set out in full in Part VIII (The Scheme of Arrangement) at the end of this document.

Nightstar retained Centerview which acted, with its affiliate Centerview UK, as financial advisor to the Nightstar Board in connection with the proposed Acquisition. The purpose of this Explanatory Statement is to explain the terms of the Acquisition and to provide you with other relevant information.

Nightstar Shareholders should read the whole of this document before deciding whether or not to vote in favour of the Scheme at the Court Meeting and the Resolutions to be proposed at the General Meeting.

The Acquisition is to be effected by way of a Court-sanctioned scheme of arrangement under Part 26 of the Companies Act 2006, which requires the approval of Nightstar Shareholders at the Court Meeting, the passing of the Resolutions at the General Meeting and the sanction of the Court.

In accordance with the terms of the Acquisition, which will be subject to the Conditions and further terms set out in Part III (Conditions to and Further Terms of the Scheme and the Acquisition) of this document, Nightstar Scheme Shareholders at the Scheme Record Time will be entitled to receive:

For each Nightstar Scheme Share,                      USD 25.50 in cash

Based on the exchange rate of approximately USD 1.3: GBP 1 at 4.00 p.m. on 5 April 2019 (being the last practicable date prior to the date of this document):

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The Nightstar Shares will be acquired by Bidco with full title guarantee, fully paid and free from all liens, equitable interests, charges, encumbrances, rights of pre-emption and any other third party rights or interests whatsoever and together with all rights existing at the date of this document or thereafter attaching thereto, including (without limitation) the right to receive and retain, in full, all dividends and other distributions (if any) declared, made or paid or any other return of capital (whether by way of reduction of share capital or share premium account or otherwise) made on or after the Effective Date in respect of the Nightstar Shares.

Biogen is focused on discovering, developing, and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases. Nightstar is a gene therapy company focused on developing novel treatments for patients suffering from rare inherited retinal diseases that would otherwise lead to blindness.

Nightstar’s lead asset is NSR-REP1 for the treatment of choroideremia (CHM), a rare, degenerative, X-linked inherited retinal disorder, which leads to blindness and has no approved treatments. CHM primarily affects males and is caused by loss of function in the CHM gene which encodes the Rab escort protein-1 (REP-1). The REP-1 protein plays a role in intracellular protein trafficking, and loss of function in the CHM gene leads to abnormal intracellular protein trafficking and impaired elimination of waste products from the retinal pigment epithelium and photoreceptors. Initially, patients with CHM experience poor night vision, and over time progressive visual loss ultimately leads to complete blindness.

NSR-REP1 is comprised of an AAV vector administered by subretinal injection which provides a functioning CHM gene and expression of the REP-1 protein to restore membrane trafficking and thereby slow, stop, or potentially reverse the decline in vision. Data from the Phase 1/2 trial of NSR-REP1 demonstrated potentially meaningful slowing of decline in visual acuity as compared to natural history as well as signs of improved visual acuity in some patients. NSR-REP1 is currently being evaluated in the ongoing Phase 3 STAR trial with data expected in the second half of 2020.

Nightstar’s second clinical program is NSR-RPGR for the treatment of XLRP, which is also a rare inherited retinal disease primarily affecting males with no approved treatments. XLRP is characterised by mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene leading to a lack of active protein transport in photoreceptors. This abnormality leads to loss of the photoreceptor cells, resulting in retinal dysfunction by adolescence and early adulthood, progressing to legal blindness when patients are in their 40s.

NSR-RPGR is comprised of an AAV vector administered by subretinal injection which provides a functioning RPGR gene and thus expression of the RPGR protein, which is critical for protein transport in photoreceptors. The restoration of photoreceptor function is intended to slow, stop, or potentially reverse the decline in vision. Phase 1/2 data from the dose escalation portion of the XIRIUS trial for NSR-RPGR demonstrated an increase in central retinal sensitivity. The Phase 2/3 dose expansion portion of the XIRIUS trial is currently ongoing.

Nightstar’s preclinical pipeline includes NSR-ABCA4 for Stargardt disease and potential programs targeting Best vitelliform macular dystrophy (Best disease) and other genetic forms of retinitis pigmentosa.

The Acquisition would bolster Biogen’s pipeline and further execute on the its strategy to develop and expand a multi-franchise neuroscience pipeline across multiple complementary modalities. Ophthalmology

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is an emerging growth area for Biogen, and the proposed acquisition of Nightstar would accelerate Biogen’s entry into ophthalmology by adding two mid- to late-stage gene therapy assets to Biogen’s pipeline, with the potential to create long-term shareholder value.

Bidder and Bidco intend to finance the Consideration payable by Bidco to Nightstar Scheme Shareholders with existing cash resources.

Nightstar is a public limited company incorporated in England and Wales with its registered office at 10 Midford Place, London W1T 5BJ, United Kingdom. Nightstar is a leading clinical-stage gene therapy company focused on developing and commercialising novel one-time treatments for patients suffering from rare inherited retinal diseases that would otherwise progress to blindness. Nightstar’s lead product candidate, NSR-REP1, is currently in Phase 3 development for the treatment of patients with choroideremia, a rare, degenerative, genetic retinal disorder that has no treatments currently available and affects approximately one in every 50,000 people. Positive results from a Phase 1/2 trial of NSR-REP1 were published in Nature Medicine in 2018, in The New England Journal of Medicine in 2016, and in The Lancet in 2014. Nightstar’s second product candidate, NSR-RPGR, is currently being evaluated in a clinical trial known as the XIRIUS trial for the treatment of patients with X-linked retinitis pigmentosa, an inherited X-linked recessive retinal disease that affects approximately one in every 40,000 people. In September 2018, Nightstar announced positive preliminary safety and efficacy data from the XIRIUS trial for the first five cohorts (combined n=15) of the dose escalation study at the EURETINA medical meeting.

Nightstar manages its business as a single operating segment, which is the business of developing and commercialising gene therapies; however, Nightstar operates in two geographic regions: the United Kingdom and the United States.

The Nightstar ADSs are traded on Nasdaq under the symbol “NITE.” Nightstar’s registered office is located at 10 Midford Place, London W1T 5BJ, United Kingdom, and its telephone number is +44 0(20) 7062 2777.

Bidder

Bidder is a limited liability company incorporated in Switzerland. Biogen, the ultimate parent company of Bidder, is a global biopharmaceutical company which is focused on discovering, developing, and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases. One of the world’s first global biotechnology companies, Biogen was founded in 1978 by Charles Weissmann, Heinz Schaller, Kenneth Murray, and Nobel Prize winners Walter Gilbert and Phillip Sharp, and today has the leading portfolio of medicines to treat multiple sclerosis, has introduced the first and only approved treatment for spinal muscular atrophy, and is focused on advancing neuroscience research programs in Alzheimer’s disease and dementia, multiple sclerosis and neuroimmunology, movement disorders, neuromuscular disorders, acute neurology, neurocognitive disorders, pain, and ophthalmology. Biogen also manufactures and commercialises biosimilars of advanced biologics.

Bidder’s registered offices are located at Neuhofstrasse 30, 6340 Baar, Switzerland and its telephone number is +41 41 728 7444. Biogen’s principal executive offices are located at 225 Binney Street, Cambridge, Massachusetts 02142 and its telephone number is +1 (617) 679-2000.

Bidco

Bidco is a newly incorporated English private limited company, and a direct, wholly-owned subsidiary of Bidder. Bidco has been formed at the direction of Bidder solely for the purposes of implementing the

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Acquisition. Bidco has not traded since its date of incorporation, nor has it entered into any obligations other than in connection with the Acquisition.

Bidco’s registered offices are located at 70 Norden Road, Maidenhead, Berkshire, SL6 4AY and its telephone number is +44 1628 501 000.

Since its inception in May 2013, Nightstar has devoted substantially all its resources to conducting preclinical studies and clinical trials, organising and staffing the company, business planning, raising capital and establishing its intellectual property portfolio. Nightstar does not have any products approved for sale and to-date, Nightstar has not generated any revenue and does not expect to generate revenue from the sale of products or any other source in the near future. If Nightstar’s development efforts for its product candidates are successful and result in regulatory approval, Nightstar may generate revenue in the future from product sales. Nightstar could also potentially generate revenue in the future from other sources including entering longer-term collaboration agreements with third parties.

Nightstar has funded its operations to date primarily with subscription proceeds from the issuance of Nightstar ADSs and Nightstar Shares. As at 31 December 2018, Nightstar had received net cash proceeds of USD 255.2 million from sales of Nightstar ADSs and Nightstar Shares. Since inception, Nightstar has incurred significant operating losses. Nightstar’s net losses were USD 36.9 million, USD 29.7 million and USD 12.2 million for the years ended 31 December 2018, 2017 and 2016, respectively. As at 31 December 2018, Nightstar had an accumulated deficit of USD 98.9 million. Nightstar expects to continue to incur significant expenses for the foreseeable future as it advances its product candidates through preclinical and clinical development, seeks regulatory approval and pursues commercialisation of any approved product candidates. In addition, if Nightstar obtains marketing approval for any of its product candidates, it expects to incur significant commercialisation expenses related to product manufacturing, marketing, sales and distribution. Nightstar’s pipeline currently contains two late stage clinical product candidates, including NSR-REP1 and NSR-RPGR, which Nightstar believes are the leading candidates in choroideremia and XLRP, respectively, and a number of earlier stage candidates at various stages of preclinical development.

As at 31 December 2018, Nightstar held cash, cash equivalents and marketable securities of USD 164.2 million.

Bidco has received irrevocable undertakings to vote or procure votes in favour of the resolutions relating to the Scheme at the Nightstar Shareholder Meetings (or in the event that the Acquisition is implemented by way of an Offer, to accept or procure acceptance of such Offer) from the Nightstar Directors, in respect of their entire beneficial holdings, amounting to 758,019 Nightstar Shares, in aggregate, representing approximately 2.3 per cent. of the issued share capital of Nightstar as at 5 April 2019, being the last practicable date prior to the publication of this document.

Bidco has also received irrevocable undertakings from Syncona Portfolio Limited and New Enterprise Associates, both Nightstar Shareholders, to vote or procure votes in favour of the resolutions relating to the Scheme at the Nightstar Shareholder Meetings (or in the event that the Acquisition is implemented by way of an Offer, to accept or procure acceptance of such Offer) in respect of their entire beneficial holdings, amounting to 19,868,280 Nightstar Shares, in aggregate, representing approximately 59.2 per cent. of the issued share capital of Nightstar as at 5 April 2019, being the last practicable date prior to the publication of this document.

The Irrevocable Undertakings given by the Nightstar Directors will cease to be binding if:

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and will remain binding if a higher competing offer is made for Nightstar.

The Irrevocable Undertakings given by Syncona Portfolio Limited and New Enterprise Associates will cease to be binding if:

and will remain binding if a higher competing offer is made for Nightstar.

The form of the Director’s Irrevocable Undertaking is attached hereto as Annex C and the form of the Shareholder’s Irrevocable Undertaking is attached hereto as Annex D. Further details of these Irrevocable Undertakings are set out in Part V (Additional Information) of this document.

Award holders in the Nightstar Share Schemes will be contacted as soon as reasonably practicable in the weeks following the date of this document regarding the effect of the Scheme on their Awards and with details of Bidder’s proposals about their rights. A summary of the effect of the Scheme on Awards is set out below.

The Scheme will apply to any Nightstar Shares which are issued pursuant to Awards (and no longer subject to forfeiture restrictions) before the Scheme Record Time. Any Nightstar Shares allotted or issued to satisfy Awards after the Scheme Record Time will, subject to the Scheme becoming Effective and the proposed amendments to the Nightstar Articles being approved at the General Meeting, be acquired by Bidco in exchange for the Consideration for each Nightstar Share so acquired.

On the Effective Date, all Awards will be cancelled and cease to be outstanding, and the holder will be entitled to receive a payment equal to the Consideration with respect to each Nightstar Share subject to the Award less any applicable exercise or purchase price and applicable withholdings.

Following the Effective Date, any Nightstar Shares subject to or issued in connection with the vesting or exercise of any Pre-IPO Equity Awards will be immediately acquired by Bidco from the relevant Award holder in exchange for the Consideration for each Nightstar Share so acquired.

Awards held by the Nightstar Directors, which consist of stock options and restricted shares, will be treated in the same way as the Awards held by other participants in the Nightstar Share Schemes.

Any Consideration payable to Award holders in the Nightstar Share Schemes will be paid to them in accordance with the terms of the Scheme, subject to any arrangements that are required to be put in place to effect the payment of any income tax and/or social security contributions that arise in connection with the Awards.

Holders of Deferred Shares will not receive the Consideration. Subject to the passing of the Re-designation Special Resolution and the Ordinary Resolution, Nightstar intends to repurchase all Deferred Shares ahead of the Effective Date under the terms of the Buy-Back Agreements.

Further information in respect of the proposed amendments to the Nightstar Articles in respect of the Scheme is contained in paragraph 10(f) below and in the Notice of General Meeting which is set out at pages ix to xiii of this document.

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It is intended that the Acquisition will be effected by way of the Scheme. The Scheme is an arrangement made between Nightstar and the Nightstar Scheme Shareholders under Part 26 of the Companies Act 2006. The provisions of the Scheme are set out in full in Part VIII (The Scheme of Arrangement) of this document. The Scheme involves an application by Nightstar to the Court to sanction the Scheme pursuant to which the Nightstar Scheme Shares will be transferred to Bidco, in consideration for which Nightstar Scheme Shareholders on the register of members of Nightstar at the Scheme Record Time will receive the Consideration. The transfer of the Nightstar Scheme Shares to Bidco, provided for in the Scheme, will result in the entire issued and to be issued share capital of Nightstar being held by Bidco and Nightstar becoming a wholly-owned subsidiary of Bidco.

Prior to the Scheme Record Time, Nightstar may allot and issue Nightstar Shares pursuant to the vesting and/or exercise of Awards under the Nightstar Share Schemes. Nightstar will not issue any shares after the Scheme Record Time until the Scheme has become Effective.

The expected timetable of principal events for the Acquisition and Scheme is set out on page xiv of this document. It is currently expected that the Scheme will become Effective mid-year 2019, subject to the satisfaction or (where applicable) waiver of all the relevant Conditions.

The Scheme is subject to the approval of Nightstar Shareholders at the Court Meeting. As at 5 April 2019, the last practicable date before this document, there were 33,536,214 Nightstar Shares issued and outstanding, held by 14 shareholders (including brokers, banks and other nominees). Nightstar Shares in which Bidder or a member of the Bidder Group is interested will not be eligible to be voted on the resolution at the Court Meeting to approve the Scheme and the Scheme will not apply to such Nightstar Shares. As at 5 April 2019 (the last practicable date prior to the publication of this document), no member of the Bidder Group held interests in Nightstar Shares.

It is important that, for the Court Meeting, as many votes as possible are cast so that the Court may be satisfied that there is a fair representation of opinion of the Nightstar Scheme Shareholders. You are therefore urged to complete and return your Form of Proxy, make an electronic appointment of a proxy or submit a proxy via CREST as soon as possible.

In addition, the Acquisition will require the approval of the Resolutions by the Nightstar Shareholders at the General Meeting. The General Meeting has been convened to consider and, if thought fit, to authorise the Nightstar Board to re-designate any Nightstar Share as a Deferred Share in respect of individuals who have entered into a Vesting Agreement with Nightstar in the manner described in paragraph 10(d) below, to pass the Resolutions to approve the terms of each of the Buy-Back Agreements available for inspection on Nightstar’s website and at its registered office, and in a form substantially identical to Annex E, to authorise the Nightstar Directors to implement the Scheme and to approve the adoption of certain amendments to the Nightstar Articles in accordance with the Scheme and in the manner described in paragraph (f) below. The General Meeting will be held shortly after the Court Meeting.

The General Meeting has been convened for 10.30 a.m. on 8 May 2019, for Nightstar Shareholders to consider and, if thought fit, pass, the Resolutions to approve:

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Voting on the Ordinary Resolution will be by poll, and each Nightstar Shareholder present, in person or by proxy, will be entitled to one vote for every Nightstar Share held as at the Scheme Voting Record Time. The approval required for the Ordinary Resolution to be passed is more than 50 per cent. of the votes cast (in person or by proxy) on the Ordinary Resolution.

Voting on the Special Resolutions will be by poll, and each Nightstar Shareholder present, in person or by proxy, will be entitled to one vote for every Nightstar Share held as at the Scheme Voting Record Time. The approval required for each Special Resolution to be passed is at least 75 per cent. of the votes cast (in person or by proxy) on such Special Resolution.

The quorum for the General Meeting will be two or more Nightstar Shareholders who held shares as at the Scheme Voting Record Time present in person or by proxy.

Nightstar Scheme Shareholders who currently hold Nightstar Shares which are proposed to be re-designated as Deferred Shares under the terms of a Vesting Agreement will not (subject to the passing of the Re-designation Special Resolution) be entitled to vote on the Ordinary Resolution or the Scheme Special Resolution.

In addition to the General Meeting, the Court Meeting has been convened for 10.40 a.m. on 8 May 2019, or, if later, as soon after that time as the General Meeting has been concluded or adjourned for Nightstar Scheme Shareholders to consider and, if thought fit, approve the Scheme.

At the Court Meeting, voting will be by poll (and not a show of hands) and each Nightstar Scheme Shareholder present, in person or by proxy, will be entitled to one vote for each Nightstar Scheme Share held as at the Scheme Voting Record Time. The approval required at the Court Meeting is a majority in number of the Nightstar Scheme Shareholders representing not less than 75 per cent. in value of the Nightstar Scheme Shares held by such Nightstar Scheme Shareholders present and voting, either in person or by proxy.

Entitlement to attend, speak and vote at these meetings and the number of votes which may be cast at the meetings will be determined by reference to the register of members of Nightstar at the Scheme Voting Record Time. Nightstar Shareholders whose names appear on the register of members of Nightstar at 6.00 p.m. on 3 May 2019 or, if the General Meeting is adjourned, on the register of members at 6.00 p.m. on the date that is 48 hours (excluding any part of a day that is not a Business Day) before the date set for the adjourned meeting, shall be entitled to attend and speak and vote at the General Meeting in respect of the number of Nightstar Shares registered in their name at the Scheme Voting Record Time. Nightstar Scheme Shareholders whose names appear on the register of members of Nightstar at 6.00 p.m. on 3 May 2019 or, if the Court Meeting is adjourned, on the register of members at 6.00 p.m. on the date that is 48 hours (excluding any part of a day that is not a Business Day) before the date set for the adjourned meeting, shall be entitled to attend and speak and vote at the Court Meeting in respect of the number of Nightstar Scheme Shares registered in their name at the Scheme Voting Record Time.

You will find the Notices of the Court Meeting and of the General Meeting set out on pages vi to viii (Notice of Court Meeting) and pages ix to xiii (Notice of General Meeting) of this document, respectively.

Under the Companies Act 2006, the Scheme also requires the sanction of the Court.

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The Court Hearing to sanction the Scheme is currently expected to take place mid-year 2019. All Nightstar Shareholders are entitled to attend the Court Hearing in person or through Counsel to support or oppose the sanctioning of the Scheme.

The Scheme will become Effective as soon as a copy of the Court Order has been delivered to the Registrar of Companies. This is currently expected to occur mid-year 2019, and no earlier than 7 June 2019. It is intended that Nightstar will be re-registered as a private limited company shortly afterwards.

If the Scheme becomes Effective, it will be binding on all Nightstar Scheme Shareholders, including any Nightstar Shareholders who did not vote to approve the Scheme or who voted against the Scheme at the Court Meeting or the General Meeting.

Unless the Scheme becomes Effective by the Long Stop Date, it will lapse and the Acquisition will not proceed (unless extended with the agreement of Bidder, Bidco, Nightstar and, if required, the approval of the Court).

The Re-designation Special Resolution to be proposed at the General Meeting authorises the Nightstar Board to re-designate certain Nightstar Shares as Deferred Shares. Holders of pre-IPO Equity Awards had certain vesting restrictions imposed on the relevant shares by the terms of the articles of association of Nightstar in force immediately prior to the IPO. The current Nightstar Articles do not contain such restrictions and holders therefore agreed contractually to re-impose these by entering into a Vesting Agreement with Nightstar.

Under the terms of the Vesting Agreements, the relevant holders agreed that upon the date such individual ceased to hold all positions as director, employee or consultant of any member of the Nightstar Group or ceased to provide services to any member of the Nightstar Group or if earlier the giving or receiving of notice of such cessation, the Nightstar Board would determine, and notify the holder of, the proportion of such holder’s shareholding that would convert to Deferred Shares.

Under Article 10 of the Nightstar Articles, the Deferred Shares are, for all practical purposes, valueless. Deferred Shares do not entitle holders to receive notice of or attend and vote at any general meeting of Nightstar or to receive a dividend or other distribution or to participate in any return of capital on a winding up (other than the nominal amount paid on such shares following a very substantial distribution of GBP 1,000,000 per Nightstar Share to the holders of Nightstar Shares).

Nightstar has not at present re-designated any such Nightstar Shares as Deferred Shares. The Nightstar Board is seeking the authority through the Re-designation Special Resolution to re-designate such Nightstar Shares as Deferred Shares as provided in the Vesting Agreements. Nightstar believes that as at the date of this document, 48,721 Nightstar Shares will be re-designated as Deferred Shares under the terms of the Vesting Agreements. Re-designation of these Nightstar Shares as Deferred Shares will not affect the rights of any other Nightstar Shareholders or Nightstar ADS Holders.

Deferred Shares are excluded from the Scheme. Consequently, any such holders of Deferred Shares as at the time of the Court Meeting are not entitled to vote at the Court Meeting notwithstanding that such holders may have received notice of the Court Meeting and are not entitled to receive any Consideration in respect of such Deferred Shares.

Under Article 10 of the Nightstar Articles, the Deferred Shares do not entitle holders to receive notice of or attend and vote at any general meeting of Nightstar or to receive a dividend or other distribution or to participate in any return of capital on a winding up (other than the nominal amount paid on such shares following a very substantial distribution of GBP 1,000,000 per Nightstar Share to the holders of Nightstar Shares). Accordingly, the Deferred Shares are, for all practical purposes, valueless. No application will be made to any exchange for admission of the Deferred Shares to listing and/or trading.

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Following the re-designation of the relevant Nightstar Shares as Deferred Shares as described in paragraph 10(d) above (conditional upon the passing of the Re-designation Special Resolution), Nightstar intends to enter into a Buy-Back Agreement with each of the individuals who will be the holders of such Deferred Shares.

Pursuant to the provisions of the Companies Act 2006, a copy of each Buy-Back Agreement will, from the date of this document, be available for inspection free of charge during normal business hours on any Business Day at Nightstar’s registered office until the date of the General Meeting. Copies of the Buy-Back Agreements will also be available for inspection at the General Meeting. The form of Buy-Back Agreement is attached hereto as Annex E.

Under the terms of each of the Buy-Back Agreements, Nightstar shall pay each such holder a consideration of GBP 0.01 for all Deferred Shares held by such holder. The Buy-Back is conditional upon Nightstar Shareholder approval. At the General Meeting, Nightstar Shareholders will be asked to approve, if thought fit, each Buy-Back Agreement pursuant to the Ordinary Resolution.

Subject to the passing of the Re-designation Special Resolution, the re-designation of the relevant Nightstar Shares as Deferred Shares as provided for under the terms of the relevant Vesting Agreement and the passing of the Ordinary Resolution, the Deferred Shares will be bought back by Nightstar and cancelled. The Buy-Back will be funded out of the proceeds of the new issue of one Nightstar Share which is being issued for the purpose of financing the Buy-Back of the Deferred Shares.

Nightstar Scheme Shareholders who currently hold Nightstar Shares which are proposed to be re-designated as Deferred Shares under the terms of a Vesting Agreement will not (subject to the passing of the Re-designation Special Resolution) be entitled to vote on the Ordinary Resolution or the Scheme Special Resolution.

The Scheme Special Resolution to be proposed at the General Meeting contains provisions to amend the Nightstar Articles to ensure that any Nightstar Shares issued (other than to Bidco or any member of the Bidder Group): (i) between the General Meeting and the Scheme Record Time will be subject to the Scheme; and (ii) after the Scheme Record Time will automatically be acquired by Bidco on the same terms as under the Scheme. These provisions will avoid any person (other than a member of the Bidder Group) holding Nightstar Shares following the Effective Date.

The Scheme contains a provision for Nightstar and Bidco to consent on behalf of all persons concerned to any modification of, or addition to, the Scheme or to any condition approved or imposed by the Court. The Court would be unlikely to approve any modification of, or additions to, or impose a condition to the Scheme which might be material to the interests of the Nightstar Shareholders unless Nightstar Shareholders were informed of such modification, addition or condition. It would be a matter for the Court to decide, in its discretion, whether or not a further meeting of Nightstar Scheme Shareholders should be held in these circumstances.

The Acquisition is subject to the Conditions and further terms set out in full in Part III (Conditions to and Further Terms of the Scheme and the Acquisition) of this document.

Amongst others, the Conditions include competition law clearances under the U.S. HSR Act and in Germany.

The Scheme is also conditional, amongst other things, upon:

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At this stage, subject to the approval and availability of the Court (which is subject to change), and subject to the satisfaction (or, where applicable, waiver) of the Conditions, Bidder expects that the Acquisition will become Effective by mid-year 2019, and no earlier than 7 June 2019.

If any Condition is not capable of being satisfied by the date specified therein, Bidder shall make an announcement through GlobeNewswire or another national news wire service as soon as practicable and, in any event, by no later than 7.00 a.m. (New York time) on the Business Day following the date so specified, stating whether Bidder has invoked that Condition, waived that Condition or, with the agreement of Nightstar, specified a new date by which that Condition must be satisfied. Further details on the Conditions are set out in Part III (Conditions to and Further Terms of the Scheme and the Acquisition) of this document.

Each of Bidder and Bidco reserves the right to elect, subject to the consent of Nightstar and subject to the terms of the Implementation Agreement, to implement the Acquisition by way of an Offer for the entire issued and to be issued share capital of Nightstar as an alternative to the Scheme. In such an event, the Offer will be implemented on the same terms (subject to appropriate amendments), so far as applicable, as those which would apply to the Scheme and subject to the amendment referred to in paragraph A of Part III (Conditions to and Further Terms of the Scheme and the Acquisition) of this document.

Summaries of the offer-related arrangements entered into in connection with the Acquisition are set out in paragraph 9 of Part V (Additional Information) of this document. These agreements have been made available on Nightstar’s website, https://ir.nightstartx.com.

Details of the interests of the Nightstar Directors in Nightstar Shares and options under the Nightstar Share Schemes are set out in paragraph 4 of Part V (Additional Information) of this document.

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The following Nightstar Directors have given irrevocable undertakings to vote in favour of the Scheme at the Court Meeting and the Resolutions to be proposed at the General Meeting as described in paragraph 8 above, in relation to the following Nightstar Shares currently held by them as well as any further Nightstar Shares they may acquire:

These Irrevocable Undertakings will continue to be binding in the event that a higher competing offer is made for Nightstar.

Bidco has also received irrevocable undertakings from Syncona Portfolio Limited and New Enterprise Associates, both Nightstar Shareholders, to vote or procure votes in favour of the resolutions relating to the Scheme at the Nightstar Shareholder Meetings (or in the event that the Acquisition is implemented by way of an Offer, to accept or procure acceptance of such Offer) in respect of their entire beneficial holdings, amounting to 19,868,280 Nightstar Shares, in aggregate, representing approximately 59.2 per cent. of the issued share capital of Nightstar as at 5 April 2019, being the last practicable date prior to publication of this document.

The Irrevocable Undertakings given by the Nightstar Directors will cease to be binding if:

and will remain binding if a higher competing offer is made for Nightstar.

The Irrevocable Undertakings given by Syncona Portfolio Limited and New Enterprise Associates will cease to be binding if:

and will remain binding if a higher competing offer is made for Nightstar.

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The form of the Director’s Irrevocable Undertaking is attached hereto as Annex C and the form of the Shareholder’s Irrevocable Undertaking is attached hereto as Annex D.

Awards held by the Nightstar Directors, which consist of stock options and restricted shares, will be treated in the same way as the other participants in the Nightstar Share Schemes.

Save as set out in this document, the effect of the Scheme on the interests of the Nightstar Directors does not differ from its effect on like interests of any other Nightstar Shareholder.

Delisting of Nightstar Shares

Prior to the Scheme becoming Effective, a request will be made by Nightstar to Nasdaq to suspend trading of, and de-list, the Nightstar ADSs on Nasdaq, to take effect on, or shortly after, the Effective Date. Subsequently, the Nightstar ADSs and Nightstar Shares will be deregistered under the U.S. Exchange Act.

As soon as practicable after the Effective Date and after the cancellation of the listing and admission to trading of the Nightstar ADSs on Nasdaq, it is intended that Nightstar will be re-registered as a private limited company under the relevant provisions of the Companies Act 2006.

Subject to the Scheme becoming Effective (and except as provided in paragraph 17 of Part II (Explanatory Statement) in relation to certain Overseas Shareholders), settlement of the Consideration to which any Nightstar Shareholder is entitled under the Scheme will be effected in the following manner:

Where, at the Scheme Record Time, a Nightstar Scheme Shareholder holds Nightstar Shares in uncertificated form, the Consideration to which such Nightstar Scheme Shareholder is entitled will be transferred to such person through CREST by Bidco procuring, via Computershare, the creation of an assured payment obligation in favour of the appropriate CREST account as soon as practicable after the Effective Date.

As from the Scheme Record Time, each holding of Nightstar Shares credited to any stock account in CREST will be disabled and all Nightstar Shares will be removed from CREST.

Bidder and Bidco reserve the right to pay all, or any part of, the Consideration referred to above in the manner referred to in sub-paragraph (ii) below if, for any reason outside Bidder and Bidco’s reasonable control, they are not able to effect payment in accordance with this sub-paragraph or if they otherwise wish to do so.

Where, at the Scheme Record Time, a Nightstar Scheme Shareholder holds Nightstar Scheme Shares in certificated form, settlement of the Consideration will be despatched:

All such cash payments will be made in U.S. dollars. Payments made by cheque will be payable to the Nightstar Scheme Shareholder concerned or, in the case of joint holders, to all joint holders on the register of members of Nightstar. Cheques will be despatched as soon as practicable after the Effective

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Date and in any event within 14 days of the Effective Date to the person entitled thereto at the address as appearing in the register of members of Nightstar at the Scheme Record Time. None of Nightstar, Bidder, Bidco, any of their nominee(s) or any of their respective agents shall be responsible for any loss or delay in the transmission of cheques sent in this way, and such cheques shall be sent entirely at the risk of the person entitled thereto.

All documents and remittances sent to, or from, by or on behalf of Nightstar Shareholders will be sent entirely at their own risk. On the Effective Date each certificate representing a holding of Nightstar Shares will cease to be a valid document of title and should be destroyed or, at the request of Nightstar, delivered to Nightstar, or to any person appointed by Nightstar to receive the same. At the Scheme Record Time entitlements to Nightstar Shares held within CREST will be disabled and all Nightstar Shares will be removed from CREST.

Settlement of the Consideration to which any Nightstar Shareholder is entitled under the Scheme will be implemented in full in accordance with the terms of the Scheme free of any lien, right of set-off, counterclaim or other analogous right to which Bidder or Bidco might otherwise be, or claim to be, entitled against such Nightstar Shareholder.

Nightstar ADS Holders will not be entitled to attend the Court Meeting or General Meeting in person or vote directly on the Acquisition. Instead, Nightstar will request that the Nightstar Depositary delivers to Nightstar ADS Holders as of the Nightstar ADS Voting Record Time a notice of (or notices for) the Court Meeting and the General Meeting, and Nightstar ADS Holders as of the Nightstar ADS Voting Record Time will have the right to instruct the Nightstar Depositary how to vote the Nightstar Shares underlying the Nightstar ADSs with respect to the Acquisition, subject to and in accordance with the terms of the Deposit Agreement, a copy of which is available free of charge at the SEC’s website at www.sec.gov or by directing a request to Nightstar’s contact for enquiries identified above. Shareholders, including ADS Holders, who hold their Nightstar Shares in the name of a broker, bank or other nominee should follow the voting instructions provided by such nominee to ensure that their Nightstar Shares are represented at the Court Meeting and the General Meeting.

On the Effective Date, Nightstar Scheme Shares as at the Scheme Record Time represented by the Nightstar ADSs will be transferred to Bidco (or such of its nominee(s) as are agreed between Bidco and Nightstar). As soon practicable after the Effective Date, the Nightstar Depositary will, in accordance with the terms of the Scheme, receive a cash amount in U.S. dollars equal to the total amount due by way of Consideration under the terms of the Scheme in respect of all the Nightstar Scheme Shares held by the Nightstar Depositary at the Scheme Record Time.

Nightstar ADS Holders as of the Nightstar ADS Voting Record Time have been sent or will be sent an ADS Voting Instruction Card and a Depositary Notice of Court Meeting and General Meeting by the Nightstar Depositary. Nightstar ADS Holders as of the Nightstar ADS Voting Record Time can direct the voting of the Nightstar Shares represented by their Nightstar ADSs, subject to the terms of the Deposit Agreement, a copy of which is available free of charge at the SEC’s website at www.sec.gov or by directing a request to Nightstar’s contact for enquiries identified above.

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Nightstar ADS Holders as at the Nightstar ADS Voting Record Time should, if they wish to direct the voting of the Nightstar Shares represented by their Nightstar ADSs held by the Nightstar Depositary (or its nominee) at the General Meeting and Court Meeting, sign, complete and return the ADS Voting Instruction Card in accordance with the instructions printed thereon and in the Depositary Notice of Court Meeting and General Meeting received from the Nightstar Depositary. The ADS Voting Instruction Card should be returned by mail to Nightstar Therapeutics plc, c/o Citibank Shareholder Services, PO Box 43099, Providence RI 02940-5000, USA, as soon as possible and, in any event, so as to be received no later than 10.00 a.m. (New York time) on 30 April 2019 or if either the Court Meeting or the General Meeting is adjourned, such later date as may be notified by the Nightstar Depositary.

Nightstar ADS Holders who hold their Nightstar ADSs indirectly (through a broker, bank or other nominee) must follow the instructions from such broker, bank or other nominee if they wish to give voting instructions to the Nightstar Depositary. Providing voting instructions via a broker, bank or other nominee may require the provision of information by a particular deadline, well in advance of the deadline to give the Nightstar Depositary voting instructions, and therefore you are encouraged to reach out to such broker, bank or other nominee as quickly as possible.

Following timely receipt of valid voting instructions from a Nightstar ADS Holder, the Nightstar Depositary shall endeavour, insofar as practicable and permitted under the provisions of, or governing, the Nightstar Shares represented by Nightstar ADSs, to vote or cause its nominee to vote (by means of the appointment of a proxy or otherwise) such Nightstar Shares represented by the Nightstar ADSs in respect of which instructions have been received in accordance with those instructions.

Nightstar ADS Holders will not be able to attend the Court Meeting or the General Meeting in person. Nightstar ADS Holders who wish to attend the Nightstar Shareholder Meetings must take steps to present their Nightstar ADSs (and, to the extent that such Nightstar ADSs are certificated, the certificates evidencing such Nightstar ADSs) to the Nightstar Depositary for cancellation before 5.00 p.m. (New York time) on 25 April 2019 (subject to the relevant Nightstar ADS Holder’s compliance with the terms of the Deposit Agreement and payment of the Nightstar Depositary’s fees), together with: (i) delivery instructions for the Nightstar Shares represented by such Nightstar ADSs (including, if applicable, the name and address of the person who will be the registered holder of such Nightstar Shares); and (ii) if the Nightstar ADS cancellation is to take place after the Nightstar ADS Voting Record Time and before the Scheme Record Time, a certification that the Nightstar ADS Holder: (x) beneficially owned the relevant Nightstar ADSs as at the Nightstar ADS Voting Record Time and has not given, and will not give, voting instructions to the Nightstar Depositary in respect of such Nightstar ADSs in relation to the Nightstar Shareholder Meetings (or has cancelled all voting instructions previously given); (y) beneficially owned the relevant Nightstar ADSs as at the Nightstar ADS Voting Record Time and has given voting instructions to the Nightstar Depositary in respect of such Nightstar ADSs in relation to the Nightstar Shareholder Meetings, but undertakes not to vote the Nightstar Shares represented by such Nightstar ADSs at the Nightstar Meetings; or (z) did not beneficially own the relevant Nightstar ADSs as at the Nightstar ADS Voting Record Time, but undertakes not to vote the Nightstar Shares represented by such Nightstar ADSs at the Nightstar Shareholder Meetings. Nightstar ADS Holders who hold their Nightstar ADSs through a broker, bank or other nominee should promptly contact their broker, bank or other nominee to find out what actions are required to cancel the Nightstar ADSs.

Nightstar ADS Holders who present their Nightstar ADSs to the Nightstar Depositary for cancellation in order to take delivery of Nightstar Shares will be responsible for the payment of the Nightstar Depositary’s fees associated with such cancellation (up to USD 0.05 per Nightstar ADS presented for cancellation).

Nightstar ADS Holders will not be permitted to cancel their Nightstar ADSs from 5.00 p.m. (New York time) on 25 April 2019 until 8.00 a.m. (New York time) on 26 April 2019. Nightstar ADS Holders who take steps (as described in the paragraphs above) to cancel their Nightstar ADSs before 5.00 p.m. (New York time) on 25 April 2019 and become Nightstar Scheme Shareholders before the Scheme Voting Record

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Time will have the right to attend the both the Nightstar Shareholder Meetings in person and be represented by counsel to support or oppose the sanctioning of the Scheme (subject to the limitations and qualifications above).

Once the Nightstar Depositary has received the relevant cash amount in U.S. dollars, each Nightstar ADS Holder will receive (upon surrender of their Nightstar ADSs to the Nightstar Depositary) their pro rata portion of the cash in U.S. dollars, net of the Nightstar Depositary fees for the cancellation of the Nightstar ADSs (up to USD 0.05 per Nightstar ADS) and the distribution of the Consideration (up to USD 0.05 per Nightstar ADS) to the surrendering Nightstar ADS Holders and applicable withholding taxes. Nightstar ADS holders who hold their Nightstar ADSs in certificated form will, on or after the Effective Date, receive letters of transmittal from the Nightstar Depositary with an explanation on how to surrender the Nightstar ADSs to the Nightstar Depositary. Those holders must sign and return the letter of transmittal, together with their Nightstar ADS certificates, to receive any consideration to which they are entitled. Nightstar ADS Holders who hold their Nightstar ADSs in uncertificated form will automatically receive Consideration to which they are entitled and do not need to take any further action. Payments to Nightstar ADS Holders will be made by cheques mailed by the Nightstar Depositary to the address the Nightstar Depositary has in its records for such registered holder.

Any Nightstar ADS Holders who hold their Nightstar ADSs indirectly through a bank, broker, or other nominee within DTC, will receive credit of the funds to their account from their bank, broker or other nominee. The Nightstar Depositary will remit the applicable funds (net of Nightstar Depositary fees and applicable withholding taxes) to DTC, and DTC will, in turn, credit the Nightstar ADS Holder’s bank, broker, securities intermediary or share plan administrator.

If Nightstar Scheme Shareholders approve the Scheme at the Court Meeting and the Court sanctions the Scheme, then, subject to the Scheme becoming Effective in accordance with its terms, the Scheme will be binding on all Nightstar Shareholders, including those who did not vote or who voted against it at the Court Meeting. If Nightstar Scheme Shareholders approve the Scheme and the Court sanctions the Scheme, no Nightstar Shareholder or Nightstar ADS Holder will have “dissenters” or “appraisal” rights or otherwise have any right to seek a court appraisal of the value of Nightstar Shares. If the Scheme becomes Effective, all Nightstar Scheme Shareholders will receive the Consideration.

This document has been prepared for the purposes of complying with English law and U.S. proxy rules and the information disclosed may not be the same as that which would have been disclosed if this document had been prepared in accordance with the laws of jurisdictions outside the United Kingdom and the United States.

This document does not constitute an offer to sell or issue or the solicitation of an offer to buy or subscribe for shares in any jurisdiction in which such offer or solicitation is unlawful.

Overseas Shareholders should consult their own legal and tax advisers with respect to the legal and tax consequences of the Scheme.

The availability of the Acquisition to Overseas Shareholders may be affected by the laws of the relevant jurisdictions in which they are located. Overseas Shareholders should inform themselves about and should observe any applicable legal or regulatory requirements. It is the responsibility of all Overseas Shareholders

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to satisfy themselves as to the full compliance of the laws of the relevant jurisdiction in connection therewith, including the obtaining of any governmental, exchange control or other consents which may be required, or the compliance with other necessary formalities which are required to be observed and the payment of any issue, transfer or other taxes due in such jurisdiction.

The release, publication or distribution of this document and/or any accompanying documents in or into or from jurisdictions other than the United Kingdom or the United States may be restricted by law and therefore any persons who are subject to the law of any jurisdiction other than the United Kingdom or the United States should inform themselves about, and observe, any applicable legal or regulatory requirements. In particular, the ability of persons who are not resident in the United Kingdom to vote their Nightstar Shares with respect to the Scheme at the Nightstar Shareholder Meetings, or to appoint another person as proxy may be affected by the laws of the relevant jurisdictions in which they are located. Any failure to comply with the applicable restrictions may constitute a violation of the securities laws of any such Restricted Jurisdiction. To the fullest extent permitted by applicable law, the companies and persons involved in the Acquisition disclaim any responsibility or liability for the violation of such restrictions by any person or any other failure to satisfy any applicable laws, regulations or requirements.

Unless otherwise determined by Bidder, and permitted by applicable law and regulation, the Acquisition will not be made available, directly or indirectly, in, into or from a jurisdiction where to do so would violate the laws in that jurisdiction and no person may vote in favour of the Acquisition by any such use, means, instrumentality or form within any jurisdiction if to do so would constitute a violation of the laws of that jurisdiction. Accordingly, copies of this document and all documents relating to the Acquisition are not being, and must not be, directly or indirectly, mailed or otherwise forwarded, distributed or sent in, into or from a jurisdiction where to do so would violate the laws in that jurisdiction, and persons receiving this document and all documents relating to the Acquisition (including custodians, nominees and trustees) must not mail or otherwise distribute or send them in, into or from such jurisdictions where to do so would violate the laws in that jurisdiction.

Persons who are not resident in the United Kingdom or the United States should inform themselves of, and observe, any applicable legal and regulatory requirements.

Copies of this document and any formal documentation relating to the Acquisition are not being, and must not be, directly or indirectly, mailed or otherwise forwarded, distributed or sent in or into or from any Restricted Jurisdiction or any jurisdiction where to do so would constitute a violation of the laws of such jurisdiction and persons receiving such documents (including custodians, nominees and trustees) must not mail or otherwise forward, distribute or send them in or into or from any Restricted Jurisdiction. Doing so may render invalid any related purported vote in respect of the Acquisition.

Neither this document nor the accompanying documents are intended to, and do not, constitute or form part of any offer or invitation to purchase, otherwise acquire, subscribe for, sell or otherwise dispose of, any securities or the solicitation of any vote or approval pursuant to the Scheme or otherwise, in any jurisdiction in which such offer, invitation or solicitation is unlawful. Nothing in this document or the accompanying documents should be relied upon for any other purpose.

This document and the accompanying documents have been prepared for the purposes of complying with English law and U.S. proxy rules and the information disclosed may not be the same as that which would have been disclosed if this document had been prepared in accordance with the laws of jurisdictions outside the United Kingdom and the United States. Overseas Shareholders should consult their own legal and tax advisers with regard to the legal and tax consequences of the Scheme for their particular circumstances.

All Nightstar Shareholders (including, without limitation, nominees, trustees or custodians) who would, or otherwise intend to, forward this document and its accompanying documents to any jurisdiction outside the United Kingdom, should seek appropriate independent professional advice before taking any action.

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The Acquisition is to be implemented by way of a scheme of arrangement under Part 26 of the Companies Act 2006. If, in the future, Bidder or Bidco exercises its right to implement the Acquisition by way of an Offer, subject to the terms of the Implementation Agreement, and determines to extend the Offer into the U.S., the Acquisition will be made in compliance with applicable U.S. laws and regulations.

It may be difficult for U.S. Nightstar Shareholders and Nightstar ADS Holders to enforce their rights and any claim arising out of the U.S. federal securities laws, because Nightstar is located in a country outside of the U.S., and some or all of its officers and directors are residents of countries outside of the U.S., the U.S. Nightstar Shareholders and Nightstar ADS Holders may not be able to sue a non-U.S. company or its officers or directors in a non-U.S. court for violations of the U.S. securities laws. Further, it may be difficult to compel a non-U.S. company and its affiliates to subject themselves to a U.S. court’s judgment.

U.S. Nightstar Shareholders and Nightstar ADS Holders also should be aware that the transaction contemplated herein may have tax consequences in the U.S. A summary of certain U.S. taxation consequences of the implementation of the Scheme for certain Nightstar Shareholders and Nightstar ADS Holders is set out in paragraph 6 of Part V (Additional Information) of this document. That summary does not constitute tax advice and does not purport to be a full analysis of all potential United States tax consequences of the Acquisition. Nightstar Shareholders and Nightstar ADS Holders who are in any doubt about their taxation position, or who are subject to taxation in a jurisdiction outside of the United Kingdom and United States are strongly advised to contact an appropriate independent professional adviser immediately.

A summary of certain United Kingdom and United States taxation consequences of the implementation of the Scheme for certain Nightstar Shareholders is set out in paragraph 6 of Part V (Additional Information) of this document.

That summary does not constitute tax advice and does not purport to be a full analysis of all potential United Kingdom and United States tax consequences of the Acquisition. Nightstar Shareholders who are in any doubt about their taxation position, or who are subject to taxation in a jurisdiction outside of the United Kingdom and the United States are strongly advised to contact an appropriate independent professional adviser immediately.

You will find enclosed with this document:

All Nightstar Shareholders:

If you hold Nightstar Shares in CREST, you may instead appoint a proxy by completing and transmitting a CREST Proxy Instruction to Nightstar’s registrars, Computershare.

If you have not received the correct documents please contact the Nightstar Shareholder Helpline on the telephone number set out on page xvii of this document.

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Forms of Proxy

Whether or not you intend to attend the Court Meeting and/or the General Meeting, please complete and sign both Forms of Proxy in accordance with the instructions printed thereon and return them by post using the prepaid envelope provided, during normal business hours only, to Computershare at The Pavilions, Bridgwater Road, Bristol, BS13 8AE, United Kingdom, so as to be received as soon as possible and in any event not later than, than 10.40 a.m. on 3 May 2019 in the case of the Court Meeting and 10.30 a.m. on 3 May 2019 in the case of the General Meeting. Your vote will be cast as specified on the applicable Form of Proxy.

If the blue Form of Proxy for the Court Meeting is not lodged by such time, it may be handed to a representative of Computershare at the venue of the Court Meeting or the Chairman of the Court Meeting before the start of the Court Meeting. However, in the case of the General Meeting, unless the white Form of Proxy is lodged so as to be received by 10.30 a.m. on 3 May 2019, it will be invalid. The completion and return of the Forms of Proxy or transmittal of a CREST Proxy Instruction will not prevent you from attending the Court Meeting or the General Meeting and voting in person, if you so wish and are so entitled.

If you hold your Nightstar Shares in uncertificated form (that is, in CREST) you may vote using the CREST Proxy Voting Service in accordance with the procedures set out in the CREST Manual (please also refer to the accompanying notes for the Notice of General Meeting set out at pages ix to xiii of this document).

Nightstar Shareholders who prefer to register the appointment of their proxy electronically via the internet can do so through the Computershare website at www.eproxyappointment.com. In order to access the voting system, shareholders will need their shareholder investor code found on their Form of Proxy. Further details in relation to electronic appointment of proxies are set out on page xxii of this document.

Proxies submitted via CREST (under CREST participant ID 3RA50) or electronically through the Computershare website must be received by Computershare not later than 10.30 a.m. on 3 May 2019 in the case of the General Meeting and 10.40 a.m. on 3 May 2019 in the case of the Court Meeting (or, in the case of an adjourned meeting, not less than 48 hours (excluding any part of a day that is not a Business Day) prior to the time and date set for the adjourned meeting).

Notices convening the Court Meeting and the General Meeting are set out on pages vi to viii (Notice of Court Meeting) and pages ix to xiii (Notice of General Meeting) of this document respectively.

It is important that as many votes as possible are cast at the Court Meeting so that the Court may be satisfied that there is a fair representation of Nightstar Scheme Shareholder opinion. You are therefore strongly encouraged to sign and return the blue Form of Proxy for the Court Meeting as soon as possible. You are also encouraged to sign and return the white Form of Proxy for the General Meeting at the same time as the blue Form of Proxy for the Court Meeting or if you hold Nightstar Shares in CREST, via a CREST Proxy Instruction.

If you have any queries relating to this document or the completion and return of the Forms of Proxy, please call the Nightstar Shareholder Helpline on +44 370 703 6033. Lines are open Monday to Friday (except public holidays) between 8.30 a.m. and 5.30 p.m.

Calls will be charged at the standard geographic rate and will vary by provider. International calls will be charged at the applicable international rate. Different charges may apply to calls from mobile telephones and calls may be recorded and randomly monitored for security and training purposes. Please note that the Nightstar Shareholder Helpline operators cannot provide advice on the merits of the Scheme or the Acquisition or give any financial, legal, investment or tax advice.

The terms of the Scheme are set out in full in Part VIII (The Scheme of Arrangement) of this document. Your attention is also drawn to the further information contained in Part III (Conditions to and Further Terms of the Scheme and the Acquisition), Part IV (Financial Information) and Part V (Additional Information).

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PART III

CONDITIONS TO AND FURTHER TERMS OF THE SCHEME AND THE ACQUISITION

Part A

Conditions to the Scheme and the Acquisition

The Acquisition is conditional upon the Scheme becoming unconditional and becoming Effective, by no later than 11.59 p.m. on the Long Stop Date.

Scheme approval

If any Condition referred to in paragraphs A(i) to A(iii) above is not capable of being satisfied by the date specified therein, Bidder shall make an announcement through GlobeNewswire or another national news wire service as soon as practicable and, in any event, by not later than 7.00 a.m. (New York time) on the Business Day following the date so specified, stating whether Bidder has invoked that Condition or, with the agreement of Nightstar, specified a new date by which that Condition must be satisfied.

In addition, Bidder and Nightstar have agreed that the Acquisition is conditional upon the following Conditions and, accordingly, the necessary actions to make the Scheme Effective will not be taken unless the following Conditions have been satisfied or, where relevant, waived:

No restraints

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Competition law and regulatory approvals

Accuracy of representations and warranties

Compliance with terms of the Implementation Agreement

No Company Material Adverse Effect

Delivery of officer’s certificate

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Part B

Waiver and Invocation of the Conditions

Each of Bidder and Bidco reserves the right to waive, in whole or in part, all or any of the Conditions in Part A above, except for Conditions A(i)(A), A(ii)(A) and A(iii)(A) (Scheme approval), which cannot be waived (save that in respect of Condition A(ii)(A), each of Bidder and Bidco continues to reserve the right to waive, in whole or in part, such Condition insofar as it relates to the passing of (i) the Ordinary Resolution; and/or (ii) the Re-designation Special Resolution).

The Acquisition is subject to the satisfaction (or waiver, if permitted) of the Conditions in Part A above, and to certain further terms set out in Part D below, and to the full terms and conditions set out in this document.

Conditions A(i), A(ii) (Scheme approval) and B (No restraints) to G (Delivery of officer’s certificate) (inclusive) must be fulfilled, determined by Bidder to be or to remain satisfied or (if capable of waiver) waived, by no later than 9.00 a.m. on the date of the Court Hearing, failing which the Acquisition will lapse. Bidder and Bidco shall be under no obligation to waive or treat as satisfied any of Conditions B (No restraints) to G (Delivery of officer’s certificate) (inclusive) by a date earlier than the latest date specified above for the fulfilment or waiver thereof, notwithstanding that the other Conditions to the Acquisition may at such earlier date have been waived or fulfilled and that there are at such earlier date no circumstances indicating that any of such Conditions may not be capable of fulfilment.

Part C

Implementation by way of an Offer

Subject to the terms of the Implementation Agreement, each of Bidder and Bidco reserves the right to elect to implement the Acquisition by way of an Offer as an alternative to the Scheme, if Nightstar provides its written consent.

In such event, such Offer will be implemented on the same terms and conditions, so far as applicable, as those which would apply to the Scheme subject to appropriate amendments to reflect the change in method of effecting the Offer, including (without limitation) the inclusion of an acceptance condition set at 90 per cent. of the Nightstar Shares to which the Offer relates or such lesser percentage as Bidder or Bidco, decides with Nightstar’s written consent, being in any case more than 50 per cent. of the Nightstar Shares to which the Offer relates.

Part D

Certain further terms of the Acquisition

The Acquisition will lapse if the Scheme does not become Effective by 11.59 p.m. on the Long Stop Date.

The availability of the Acquisition to persons not resident in the U.K. or U.S. may be affected by the Laws of the relevant jurisdictions. Persons who are not resident in the U.K. or U.S. should inform themselves about, and observe, any applicable requirements. Nightstar Shareholders who are in any doubt about such matters should consult an appropriate independent professional adviser in the relevant jurisdiction without delay and observe any applicable requirements.

This Acquisition will be governed by English law and be subject to the jurisdiction of the English courts.

Each of the Conditions shall be regarded as a separate Condition and shall not be limited by reference to any other Condition.

The Nightstar Shares will be acquired by Bidco (or its nominee) with full title guarantee, fully paid and free from all liens, equitable interests, charges, encumbrances, rights of pre-emption and any other third party rights or interests whatsoever and together with all rights existing at the date of the Announcement or thereafter attaching thereto, including (without limitation) the right to receive and retain, in full, all dividends and other distributions (if any) declared, made or paid or any other return of capital (whether by way of reduction of share capital or share premium account or otherwise) made on or after the Effective Date in respect of the Nightstar Shares.

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PART IV

FINANCIAL INFORMATION

Part A: Financial Information relating to Nightstar

The following sets out financial information in respect of Nightstar. The documents referred to below, the contents of which have previously been filed with the SEC, are incorporated into this document by reference:

Part B: Financial Information relating to the Biogen Group

The following sets out financial information in respect of Biogen. The documents referred to below, the contents of which have previously been filed with the SEC, are incorporated into this document by reference:

No incorporation of website information

Save as expressly referred to herein, neither the content of Biogen’s or Nightstar’s websites, nor the contents of any website accessible from hyperlinks from Biogen’s or Nightstar’s website, is incorporated into, or forms part of, this document.

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PART V

ADDITIONAL INFORMATION

The registered office of Nightstar and the business address of each of the Nightstar Directors is 10 Midford Place, London, W1T 5BJ.