Sec Form 4 Filing - KANIA EDWIN M JR @ ACCELERON PHARMA INC - 2014-05-05

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person KANIA EDWIN M JR 2. Issuer Name and Ticker or Trading Symbol ACCELERON PHARMA INC [ XLRN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O FLAGSHIP VENTURES, 1 MEMORIAL DRIVE 3. Date of Earliest Transaction (MM/DD/YY) 05/05/2014 (Street) CAMBRIDGE, MA02142 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 05/05/2014 S 202,879 D $ 42.86 1,000,841 I See Footnote ( 1 ) Common Stock 05/05/2014 S 12,263 D $ 42.86 60,496 I See Footnote ( 2 ) Common Stock 4,867 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other KANIA EDWIN M JR C/O FLAGSHIP VENTURES, 1 MEMORIAL DRIVE CAMBRIDGE, MA02142 X Signatures /s/ Edwin M. Kania, Jr. 05/06/2014 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents shares held by Applied Genomic Technology Capital Fund, L.P. ("AGTC Fund"). NewcoGen Group, Inc. ("NewcoGen Inc.") is the general partner of AGTC Partners, L.P., which is the general partner of AGTC Fund. NewcoGen Inc. is a wholly-owned subsidiary of Flagship Ventures Management, Inc. Noubar B. Afeyan, Ph.D. and Mr. Kania are the directors of Flagship Ventures Management, Inc. and, as such, have shared voting and dispositive power over all shares held by AGTC Fund. Each of Mr. Kania and Dr. Afeyan disclaim beneficial ownership over the shares held by AGTC Fund except to the extent of his pecuniary interest therein. ( 2 )Represents shares held by AGTC Advisors Fund, L.P. ("AGTC Advisors"). NewcoGen Inc. is the general partner of AGTC Partners, L.P., which is the general partner of AGTC Advisors. NewcoGen Inc. is a wholly-owned subsidiary of Flagship Ventures Management, Inc. Noubar B. Afeyan, Ph.D. and Mr. Kania are the directors of Flagship Ventures Management, Inc. and, as such, have shared voting and dispositive power over all shares held by AGTC Advisors. Each of Mr. Kania and Dr. Afeyan disclaim beneficial ownership over the shares held by AGTC Advisors except to the extent of his pecuniary interest therein.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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