UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 14A
Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No.     )
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Acceleron Pharma Inc.
(Name of Registrant as Specified In Its Charter)
(Name of Person(s) Filing Proxy Statement, if other than the Registrant)
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See the reverse side of this notice to obtain proxy materials and voting instructions. *** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Shareholder Meeting to Be Held on <mtgdate>. You are receiving this communication because you hold shares in the above named company. This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side). We encourage you to access and review all of the important information contained in the proxy materials before voting. Meeting Information Meeting Type: <mtgtype> For holders as of: <recdate> Date: Time: <mtgtime> Location: 0000213264_1 R1.0.0.51160 ACCELERON PHARMA INC. ACCELERON PHARMA INC. 128 SIDNEY STREET CAMBRIDGE, MA 02139 Annual Meeting April 21, 2014 June 20, 2014 June 20, 2014 11:00 AM EDT Ropes & Gray LLP Prudential Tower 800 Boylston Street Boston, MA 02199

Please Choose One of the Following Voting Methods Vote In Person: Many shareholder meetings have attendance requirements including, but not limited to, the possession of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any special requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares. Vote By Internet: To vote now by Internet, go to www.proxyvote.com. Have the information that is printed in the box marked by the arrow available and follow the instructions. Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card. How To Vote . XXXX XXXX XXXX Before You Vote How to Access the Proxy Materials Proxy Materials Available to VIEW or RECEIVE: How to View Online: Have the information that is printed in the box marked by the arrow (located on the following page) and visit: www.proxyvote.com. How to Request and Receive a PAPER or E-MAIL Copy: If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET: www.proxyvote.com 2) BY TELEPHONE: 1-800-579-1639 3) BY E-MAIL*: sendmaterial@proxyvote.com * If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked by the arrow (located on the following page) in the subject line. . XXXX XXXX XXXX . XXXX XXXX XXXX 0000213264_2 R1.0.0.51160 1. Form 10-K 2. Notice & Proxy Statement Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before June 04, 2014 to facilitate timely delivery.

Voting items 0000213264_3 R1.0.0.51160 The Board of Directors recommends you vote FOR the following: 1. Election of Directors Nominees 01 Jean M. George 02 George Golumbeski, PhD 03 Edwin M. Kania, Jr. The Board of Directors recommends you vote FOR the following proposal: 2 To ratify the selection of Ernst & Young LLP as the Company's independent registered public accounting firm for 2014. NOTE: Such other business as may properly come before the meeting or any adjournment thereof.

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