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 OF SECURITIES
                                                                                  
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Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940
                      

1. Name and Address of Reporting Person *

Polaris Venture Management Co IV LLC 		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 POLARIS VENTURE PARTNERS, 1000 WINTER STREET SUITE 3350 		3. Date of Earliest Transaction (MM/DD/YYYY)

5/5/2014
(Street)

WALTHAM, MA 02451
(City)        (State)        (Zip)
4. If Amendment, Date Original Filed (MM/DD/YYYY)

 
6. Individual or Joint/Group Filing (Check Applicable Line)

___ Form filed by One Reporting Person
_ X _ Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3) 		2. Trans. Date 2A. Deemed Execution Date, if any 3. Trans. 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   5/5/2014     S    313491   (1) (2) D $42.86   2819743   (3) I   (3) See footnote   (3)

Table II - Derivative Securities Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities)
1. Title of Derivate Security
(Instr. 3) 		2. Conversion or Exercise Price of Derivative Security 3. Trans. Date 3A. Deemed Execution Date, if any 4. Trans. 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 7. Title and Amount of Securities Underlying Derivative Security
(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

Explanation of Responses:
( 1)  Represents 307,893 shares and 5,598 shares that were held directly by Polaris Venture Partners IV, L.P. ("PVP IV") and Polaris Venture Partners Entrepreneurs Fund IV, L.P. ("PVPE IV" and, together with PVP IV, the "Funds"), respectively. Polaris Venture Management Co. IV, L.L.C. ("PVM IV") is the general partner of the Funds. PVM IV disclaims beneficial ownership of these 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.
( 2)  (Continued from footnote 1) Each of Jonathan A. Flint, Terrance G. McGuire and Alan G. Spoon are the managing members of PVM IV. Each of Messrs. Flint, McGuire and Spoon disclaims beneficial ownership of these 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.
( 3)  Represents 2,769,495 shares and 50,248 shares that are held directly by PVP IV and PVPE IV, respectively. PVM IV disclaims beneficial ownership of these 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. Each of Jonathan A. Flint, Terrance G. McGuire and Alan G. Spoon are the managing members of PVM IV. Each of Messrs. Flint, McGuire and Spoon disclaims beneficial ownership of these 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.

Reporting Owners
Reporting Owner Name / Address
Relationships
Director 10% Owner Officer Other
Polaris Venture Management Co IV LLC
C/O POLARIS VENTURE PARTNERS
1000 WINTER STREET SUITE 3350
WALTHAM, MA 02451
X

POLARIS VENTURE PARTNERS ENTREPRENEURS FUND IV LP
C/O POLARIS VENTURE PARTNERS
1000 WINTER STREET SUITE 3350
WALTHAM, MA 02451
X

POLARIS VENTURE PARTNERS IV LP
C/O POLARIS VENTURE PARTNERS
1000 WINTER STREET SUITE 3350
WALTHAM, MA 02451
X


Signatures
Polaris Venture Management Co. IV, L.L.C., By: /s/ Terrance McGuire 5/6/2014
** Signature of Reporting Person Date

Polaris Venture Partners IV, L.P., By: /s/ Terrance McGuire 5/6/2014
** Signature of Reporting Person Date

Polaris Venture Partners Entrepreneurs Fund IV, L.P., By: /s/ Terrance McGuire 5/6/2014
** Signature of Reporting Person Date


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 control number.
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