X0306 4 2016-12-07 0 0001280600 ACCELERON PHARMA INC XLRN 0001117003 MCGUIRE TERRANCE ONE MARINA PARK DRIVE, SUITE 1100 BOSTON MA 02210 1 0 0 0 Common Stock 2016-12-07 4 X 0 180518 5.88 A 778671 I See Footnote Common Stock 2016-12-08 4 S 0 34285 30.96 D 744386 I See Footnote Common Stock 2016-12-07 4 X 0 3384 5.88 A 747770 I See Footnote Common Stock 2016-12-08 4 S 0 643 30.96 D 747127 I See Footnote Common Stock 1624 D Warrant (Right to Buy) 5.88 2016-12-07 4 X 0 180518 0.00 D 2010-06-10 2020-06-10 Common Stock 180518 0 I See Footnote Warrant (Right to Buy) 5.88 2016-12-07 4 X 0 3384 0.00 D 2010-06-10 2020-06-10 Common Stock 3384 0 I See Footnote Represents 766,574 shares, 10,102 shares, and 1,995 shares directly owned by Polaris Venture Partners IV, L.P. ("PVP IV"), Polaris Venture Partners Entrepreneurs' Fund IV, L.P. ("PVPE IV") and Polaris Venture Management IV, L.L.C. ("PVM IV"), respectively. The Reporting Person is a managing member of PVM IV, the general partner of each of PVP IV and PVPE IV. The Reporting Person disclaims beneficial ownership of the Funds' and PVM IV's securities and this report shall not be deemed an admission that any of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interest, if any, therein. On December 7, 2016, the PVP IV exercised a warrant to purchase 180,518 share of the Issuer's Common Stock for $5.88 per share. PVP IV paid the exercise price on a cashless basis, resulting in the Issuer withholding 34,285 shares of the Issuer's shares of common stock subject to the warrant to pay the exercise price and issuing PVP IV the remaining 146,233 shares of the Issuer's common stock. The Issuer also paid $17.76 to PVP IV in lieu of a fractional share. Represents 732,289 shares, 10,102 shares, and 1,995 shares directly owned by PVP IV, PVPE IV and PVM IV, respectively. Represents 732,289 shares, 13,486 shares, and 1,995 shares directly owned by PVP IV, PVPE IV and PVM IV, respectively. On December 7, 2016, the PVPE IV exercised a warrant to purchase 3,384 share of the Issuer's Common Stock for $5.88 per share. PVPE IV paid the exercise price on a cashless basis, resulting in the Issuer withholding 643 shares of the Issuer's shares of common stock subject to the warrant to pay the exercise price and issuing PVPE IV the remaining 2,741 shares of the Issuer's common stock. The Issuer also paid $9.36 to PVPE IV in lieu of a fractional share. Represents 732,289 shares, 12,843 shares, and 1,995 shares directly owned by PVP IV, PVPE IV and PVM IV, respectively. /s/ Mary Blair, Attorney in-Fact for Terrance McGuire 2016-12-09