Stocks hit session lows Friday afternoon, with the S&P down 0.75% and the Nasdaq lower by 0.5% -- despite huge gains in Amazon.com (NASDAQ: AMZN).

Biotech was among the worst performing sectors with ETFs like iShares Nasdaq Biotechnology (NASDAQ: IBB) falling 2.75%, largely due to steep losses in Biogen (NASDAQ: BIIB) after it cut FY guidance.

Housing was another area of weakness, with SPDR S&P Homebuilders ETF falling following disappointing new home sales data.