Sec Form 3 Filing - ATV Entrepreneurs VII, L.P. @ ACCELERON PHARMA INC - 2013-09-18

FORM 3 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

OMB APPROVAL OMB Number: 3235-0287 Expires: November 30, 2011 Estimated average burden hours per response... 0.5

1. Name and Address of Reporting Person ATV Entrepreneurs VII, L.P. 2. Issuer Name and Ticker or Trading Symbol ACCELERON PHARMA INC [ XLRN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O ADVANCED TECHNOLOGY VENTURES, 500 BOYLSTON STREET SUITE 1380 3. Date of Earliest Transaction (MM/DD/YY) 09/18/2013 (Street) BOSTON, MA02116 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 9,732 D ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 )

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 Series A Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 1,376,248 D ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 ) ( 10 ) Series B Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 507,664 D ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 ) Series C Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 293,142 D ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 ) Series D Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 39,172 D ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 ) Series E Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 223,806 D ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 ) Series F Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 64,019 D ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 ) Warrants to Purchase Common Stock $ 5.88 ( 11 ) ( 11 ) Common Stock 148,309 D ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other ATV Entrepreneurs VII, L.P. C/O ADVANCED TECHNOLOGY VENTURES 500 BOYLSTON STREET SUITE 1380 BOSTON, MA02116 X Signatures /s/ Douglas Pagan, attorney-in-fact for ATV Entrepreneurs VII, L.P. 09/19/2013 Signature of Reporting Person Date

Explanation of Responses:

( 1 )7,831 shares of common stock, 119,322 warrants to purchase common stock, 1,103,378 shares of Series A Convertible Preferred Stock, 408,439 shares of Series B Convertible Preferred Stock, 235,847 shares of Series C Convertible Preferred Stock, 31,518 shares of Series D Convertible Preferred Stock, 111,722 shares of Series E Convertible Preferred Stock and 51,507 shares of Series F Convertible Preferred Stock of the registrant are owned directly by Advanced Technology Ventures VII, L.P. (ATV VII). ( 2 )ATV Associates VII, L.L.C. (ATV A VII) is the general partner of ATV VII and exercises voting and dispositive authority over the shares held by ATV VII. Jean George (George) is a managing director of ATV A VII and exercises voting and dispositive decisions of ATV A VII collectively with each of four other managing directors. ATV A VII and each of the managing directors disclaim beneficial ownership of these securities and this report shall not be deemed an admission that they are the beneficial owners of such securities for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interest therein. ( 3 )314 shares of common stock, 4,788 warrants to purchase common stock, 44,278 shares of Series A Convertible Preferred Stock, 16,390 shares of Series B Convertible Preferred Stock, 9,464 shares of Series C Convertible Preferred Stock, 1,264 shares of Series D Convertible Preferred Stock, 4,483 shares of Series E Convertible Preferred Stock and 2,067 shares of Series F Convertible Preferred Stock of the registrant are owned directly by Advanced Technology Ventures VII (B), L.P. (ATV VII B). ( 4 )ATV A VII is the general partner of ATV VII B and exercises voting and dispositive authority over the shares held by ATV VII B. ATV A VII and each of the managing directors disclaim beneficial ownership of these securities and this report shall not be deemed an admission that they are the beneficial owners of such securities for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interest therein. ( 5 )151 shares of common stock, 2,301 warrants to purchase common stock, 2 1,283 shares of Series A Convertible Preferred Stock, 7,878 shares of Series B Convertible Preferred Stock, 4,549 shares of Series C Convertible Preferred Stock, 607 shares of Series D Convertible Preferred Stock, 2,155 shares of Series E Convertible Preferred Stock and 993 shares of Series F Convertible Preferred Stock of the registrant are owned directly by Advanced Technology Ventures VII (C), L.P. (ATV VII C). ATV A VII is the general partner of ATV VII C and exercises voting and authority decisions over the shares held by ATV VII C. ATV A VII and each of the managing directors disclaim beneficial ownership of these securities and this report shall not be deemed an admission that they are the beneficial owners of such securities for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interest therein. ( 6 )46 shares of common stock, 711 warrants to purchase common stock, 6,575 shares of Series A Convertible Preferred Stock, 2,434 shares of Series B Convertible Preferred Stock, 1,405 shares of Series C Convertible Preferred Stock, 187 shares of Series D Convertible Preferred Stock, 665 shares of Series E Convertible Preferred Stock and 307 shares of Series F Convertible Preferred Stock of the registrant are owned directly by ATV Entrepreneurs VII, L.P. (ATV VII E). ATV A VII is the general partner of ATV VII E and exercises voting and dispositive authority over the shares held by ATV VII E. ATV A VII and each of the managing directors disclaim beneficial ownership of these securities and this report shall not be deemed an admission that they are the beneficial owners of such securities for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interest therein. ( 7 )1,307 shares of common stock, 19,916 warrants to purchase common stock, 184,810 shares of Series A Convertible Preferred Stock, 68,172 shares of Series B Convertible Preferred Stock, 39,365 shares of Series C Convertible Preferred Stock, 5,260 shares of Series D Convertible Preferred Stock, 18,647 shares of Series E Convertible Preferred Stock and 8,597 shares of Series F Convertible Preferred Stock of the registrant are owned directly by Advanced Technology Ventures VI, L.P. (ATV VI). ATV Associates VI, L.L.C. (ATV A VI) is the general partner of ATV VI and exercises voting and dispositive authority over the shares held by ATV VI. ATV A VI and each of the managing directors disclaim beneficial ownership of these securities and this report shall not be deemed an admission that they are the beneficial owners of such securities for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interest therein. ( 8 )83 shares of common stock, 1,271 warrants to purchase common stock, 11,796 shares of Series A Convertible Preferred Stock, 4,351 shares of Series B Convertible Preferred Stock, 2,512 shares of Series C Convertible Preferred Stock, 336 shares of Series D Convertible Preferred Stock, 1,190 shares of Series E Convertible Preferred Stock and 548 shares of Series F Convertible Preferred Stock of the registrant are owned directly by ATV Entrepreneurs VI, L.P. (ATV VI E). ATV A VI is the general partner of ATV VI E and exercises voting and dispositive decisions over the shares held by ATV VI E. ATV A VI and each of the managing directors disclaim beneficial ownership of these securities and this report shall not be deemed an admission that they are the beneficial owners of such securities for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interest therein. ( 9 )The Series A Convertible Preferred Stock, Series B Convertible Preferred Stock, Series C Convertible Preferred Stock, Series D Convertible Preferred Stock, Series E Convertible Preferred Stock and Series F Convertible Preferred Stock are each convertible into Common Stock at any time at the holders election and upon the consummation of a qualified public offering of the registrant. There is no expiration date for any series of convertible preferred stock. ( 10 )4,128 shares of Series A Convertible Preferred Stock of the registrant are owned directly by ATV Alliance 2003, L.P. (ATV 2003). ATV Alliance Associates, L.L.C. (ATV Alliance LLC) is the general partner of ATV 2003 and exercises voting and dispositive authority over the shares held by ATV 2003. Voting and dispositive decisions of ATV Alliance LLC are made by Ms. George. ATV Alliance LLC and George disclaim beneficial ownership of these securities and this report shall not be deemed an admission that they are the beneficial owners of such securities for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interest therein. ( 11 )The warrants to purchase Common Stock are exercisable at any time at the holders election, and have an expiration date on the ten year anniversary of the warrant issue date.

Remarks: Exhibit List Exhibit 24 Power of Attorney

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).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.