Form SCHEDULE 13G Q32 Bio Inc. Filed by: RA CAPITAL MANAGEMENT, L.P.

The address of the principal business office of each of the Reporting Persons is: c/o RA Capital Management, L.P., 200 Berkeley Street, 18th Floor, Boston MA 02116

RA Capital and the Fund are Delaware limited partnerships. Dr. Kolchinsky and Mr. Shah are United States citizens.

Row 9 of each Reporting Person's cover page to this Schedule 13G sets forth the aggregate number of shares of common stock of the Issuer beneficially owned by such Reporting Person and is incorporated by reference. The Fund directly holds 2,332,457 shares of the Issuer's common stock. RA Capital Healthcare Fund GP, LLC is the general partner of the Fund. The general partner of RA Capital is RA Capital Management GP, LLC, of which Dr. Kolchinsky and Mr. Shah are the controlling persons. RA Capital serves as investment adviser for the Fund and may be deemed a beneficial owner, for purposes of Section 13(d) of the Act, of any securities of the Issuer held by the Fund. The Fund has delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in the Fund's portfolio, including the shares of the Issuer's common stock reported herein. Because the Fund has divested voting and investment power over the reported securities it holds and may not revoke that delegation on less than 61 days' notice, the Fund disclaims beneficial ownership of the securities it holds for purposes of Section 13(d) of the Act. As managers of RA Capital, Dr. Kolchinsky and Mr. Shah may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Issuer beneficially owned by RA Capital. RA Capital, Dr. Kolchinsky, and Mr. Shah disclaim beneficial ownership of the securities reported in this Schedule 13G other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13G shall not be deemed an admission that either RA Capital, Dr. Kolchinsky, or Mr. Shah is the beneficial owner of such securities for any other purpose.

Row 11 of each Reporting Person's cover page to this Schedule 13G sets forth the percentages of shares of common stock of the Issuer beneficially owned by such Reporting Person and is incorporated by reference. Such percentage is based upon the sum of (i) 16,956,415 shares of common stock outstanding as of May 1, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission (the "SEC") on May 5, 2026 and (ii) 6,725,000 shares of common stock issued in the private placement of equity securities by the Issuer that closed on May 28, 2026.  %

Row 5 of each Reporting Person's cover page to this Schedule 13G sets forth the sole power to vote or to direct the vote of securities of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.

Row 6 of each Reporting Person's cover page to this Schedule 13G sets forth the shared power to vote or to direct the vote of securities of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.

Row 7 of each Reporting Person's cover page to this Schedule 13G sets forth the sole power to dispose or to direct the disposition of securities of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.

Row 8 of each Reporting Person's cover page to this Schedule 13G sets forth the shared power to dispose or to direct the disposition of securities of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.