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Filing Details

Issuer

Cik	Name	Symbol	Sector (SIC)	IRS No
1280600	Acceleron Pharma Inc	XLRN	Biological Products, (No Disgnostic Substances) (2836)	000000000

Insiders

Cik	Name	Reported Address	Insider Title	Director	Officer	Large Shareholder	Other
1117003	Terrance Mcguire	C/O Polaris Venture Partners 1000 Winter Street Waltham MA 02451		Yes	No	No	No

Reported Non-Derivative Transactions

Sec. Name	Acquisiton - Disposition	Date	Amount	Price	Remaning Holdings	Equity Swap Involved	Form Type	Code	Nature of Ownership	Explanation
Common Stock	Disposition	2015-08-17	407,259	$30.25	1,303,232	No	4	S	Indirect	See Footnotes

Equity Swap Involved	Form Type	Code	Nature of Ownership	Explanation
No	4	S	Indirect	See Footnotes

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.