X0306 4 2015-08-17 0 0001280600 ACCELERON PHARMA INC XLRN 0001117003 MCGUIRE TERRANCE C/O POLARIS VENTURE PARTNERS 1000 WINTER STREET WALTHAM MA 02451 1 0 0 0 Common Stock 2015-08-17 4 S 0 407259 30.25 D 1303232 I See Footnotes Represents 400,000 shares sold by Polaris Venture Partners IV, L.P. ("PVP IV") and 7,259 shares sold by Polaris Venture Partners Entrepreneurs' Fund IV, L.P. ("PVPE IV" and, together with PVP IV, the "Funds"). The Reporting Person is a director of the Issuer and managing member of Polaris Venture Management Co. IV, L.L.C. ("PVM IV"), the general partner of each of PVP IV and PVPE IV. The amount of securities beneficially owned as reported in column 5 represents 1,273,175 shares, 22,983 shares, and 7,074 shares that are held directly by PVP IV, PVPE IV and PVM IV, respectively following the transactions reported in this Form 4. PVM IV disclaims beneficial ownership of the Funds' securities and this report shall not be deemed an admission that PVM IV is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of its pecuniary interest, if any, therein. (Continued in Footnote 4) Each of Jonathan A. Flint and the Reporting Person are the managing members of PVM IV and each of them 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. /s/ Terrance McGuire 2015-08-19