SCHEDULE 13G Amendment No. 1 ACCELERON PHARMA INC COMMON STOCK Cusip #00434H108 Cusip #00434H108 Item 1: Reporting Person - FMR LLC Item 4: Delaware Item 5: 0 Item 6: 0 Item 7: 4,206,928 Item 8: 0 Item 9: 4,206,928 Item 11: 13.376% Item 12: HC Cusip #00434H108 Item 1: Reporting Person - Edward C. Johnson 3d Item 4: United States of America Item 5: 0 Item 6: 0 Item 7: 4,206,928 Item 8: 0 Item 9: 4,206,928 Item 11: 13.376% Item 12: IN SCHEDULE 13G - TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b) Item 1(a). Name of Issuer: ACCELERON PHARMA INC Item 1(b). Name of Issuer's Principal Executive Offices: 128 SYDNEY STREET Cambridge, MA 02139MA USA Item 2(a). Name of Person Filing: FMR LLC Item 2(b). Address or Principal Business Office or, if None, Residence: 245 Summer Street, Boston, Massachusetts 02210 Item 2(c). Citizenship: Not applicable Item 2(d). Title of Class of Securities: COMMON STOCK Item 2(e). CUSIP Number: 00434H108 Item 3. This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b) and the person filing, FMR LLC, is a parent holding company in accordance with Section 240.13d- 1(b)(ii)(G). (Note: See Item 7). Item 4. Ownership (a) Amount Beneficially Owned: 4,206,928 (b) Percent of Class: 13.376% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote: 0 (ii) shared power to vote or to direct the vote: 0 (iii) sole power to dispose or to direct the disposition of: 4,206,928 (iv) shared power to dispose or to direct the disposition of: 0 Item 5. Ownership of Five Percent or Less of a Class. Not applicable. Item 6. Ownership of More than Five Percent on Behalf of Another Person. Various persons have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the COMMON STOCK of ACCELERON PHARMA INC . The interest of one person, Select Biotechnology Portfolio, an investment company registered under the Investment Company Act of 1940, in the COMMON STOCK of ACCELERON PHARMA INC , amounted to 2,823,620 shares or 8.978% of the total outstanding COMMON STOCK at April 30, 2014. Item 7. Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company. See attached Exhibit A. Item 8. Identification and Classification of Members of the Group. Not applicable. See attached Exhibit A. Item 9. Notice of Dissolution of Group. Not applicable. Item 10. Certification. By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired in the ordinary course of business and were not acquired for the purpose of and do not have the effect of changing or influencing the control of the issuer of such securities and were not acquired in connection with or as a participant in any transaction having such purpose or effect. Signature After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. May 9, 2014 Date /s/ Scott C. Goebel Signature Scott C. Goebel Duly authorized under Power of Attorney effective as of June 1, 2008, by and on behalf of FMR LLC and its direct and indirect subsidiaries SCHEDULE 13G - TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b) Pursuant to the instructions in Item 7 of Schedule 13G, Fidelity Management & Research Company ("Fidelity"), 245 Summer Street, Boston, Massachusetts 02210, a wholly-owned subsidiary of FMR LLC and an investment adviser registered under Section 203 of the Investment Advisers Act of 1940, is the beneficial owner of 775,800 shares or 2.467% of the COMMON STOCK outstanding of ACCELERON PHARMA INC ("the Company") as a result of acting as investment adviser to various investment companies registered under Section 8 of the Investment Company Act of 1940. The ownership of one investment company, Select Biotechnology Portfolio, amounted to 2,823,620 shares or 8.978% of the COMMON STOCK outstanding. Select Biotechnology Portfolio has its principal business office at 245 Summer Street, Boston, Massachusetts 02210. Edward C. Johnson 3d and FMR LLC, through its control of Fidelity, and the funds each has sole power to dispose of the 775,800 shares owned by the Funds. Fidelity SelectCo, LLC ("SelectCo"), 1225 17th Street, Suite 1100, Denver, Colorado 80202, a wholly-owned subsidiary of FMR LLC and an investment adviser registered under Section 203 of the Investment Advisers Act of 1940, is the beneficial owner of 3,431,128 shares or 10.910% of the COMMON STOCK outstanding of ACCELERON PHARMA INC ("the Company") as a result of acting as investment adviser to various investment companies registered under Section 8 of the Investment Company Act of 1940 (the "SelectCo Funds"). Edward C. Johnson 3d and FMR LLC, through its control of SelectCo, and the SelectCo Funds each has sole power to dispose of the 3,431,128 owned by the SelectCo Funds. Members of the family of Edward C. Johnson 3d, Chairman of FMR LLC, are the predominant owners, directly or through trusts, of Series B voting common shares of FMR LLC, representing 49% of the voting power of FMR LLC. The Johnson family group and all other Series B shareholders have entered into a shareholders' voting agreement under which all Series B voting common shares will be voted in accordance with the majority vote of Series B voting common shares. Accordingly, through their ownership of voting common shares and the execution of the shareholders' voting agreement, members of the Johnson family may be deemed, under the Investment Company Act of 1940, to form a controlling group with respect to FMR LLC. Neither FMR LLC nor Edward C. Johnson 3d, Chairman of FMR LLC, has the sole power to vote or direct the voting of the shares owned directly by the Fidelity Funds, which power resides with the Funds' Boards of Trustees. Fidelity carries out the voting of the shares under written guidelines established by the Funds' Boards of Trustees. SCHEDULE 13G - TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b) RULE 13d-1(f)(1) AGREEMENT The undersigned persons, on May 9, 2014, agree and consent to the joint filing on their behalf of this Schedule 13G in connection with their beneficial ownership of the COMMON STOCK of ACCELERON PHARMA INC at April 30, 2014. FMR LLC By /s/ Scott C. Goebel Scott C. Goebel Duly authorized under Power of Attorney effective as of June 1, 2008, by and on behalf of FMR LLC and its direct and indirect subsidiaries Edward C. Johnson 3d By /s/ Scott C. Goebel Scott C. Goebel Duly authorized under Power of Attorney effective as of June 1, 2008, by and on behalf of Edward C. Johnson 3d Fidelity Management & Research Company By /s/ Scott C. Goebel Scott C. Goebel Senior V.P. and General Counsel Select Biotechnology Portfolio By /s/ Scott C. Goebel Scott C. Goebel Secretary Document-Separator. This page separates the filing documents of two notifications. Page of Exhibit A