SC 13D

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No.     )*

Acceleron Pharma Inc.

(Name of Issuer)

Common Stock, $0.001 par value

(Title of Class of Securities)

00434H108

(CUSIP Number)

Robert J. Hugin

Chairman of the Board,

President and Chief Executive Officer

Celgene Corporation

86 Morris Avenue

Summit, New Jersey 07901

(908) 673-9000

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

September 24, 2013

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition which is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g) check the following box: ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

SCHEDULE 13D

CUSIP No. 00434H108

1	NAME OF REPORTING PERSON: I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY) CELGENE CORPORATION 22-2711928
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS* WC
5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)  ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware, U.S.A.
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER -3,211,866-
		8	SHARED VOTING POWER -0-
		9	SOLE DISPOSITIVE POWER -3,211,866-
		10	SHARED DISPOSITIVE POWER -0-
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 3,211,866
12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*  ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 11.4% (1)
14	TYPE OF REPORTING PERSON* CO

(1) The percentage ownership is based on 28,086,834 shares of Common Stock outstanding as of September 24, 2013, which includes (i) 20,964,188 shares of Common Stock outstanding as of June 30, 2013, as reported by Acceleron in its prospectus filed with the Securities and Exchange Commission on September 19, 2013 (the “Prospectus”), (ii) 6,417,000 shares of Common Stock sold under the Prospectus (including 837,000 shares of Common Stock sold upon the exercise in full by the underwriters of their option to purchase additional shares to cover over-allotments), and (iii) 38,979 shares of Common Stock underlying a warrant held by Celgene Corporation that is exercisable immediately.

SCHEDULE 13D

Acceleron Pharma Inc.

Item 1. Security and Issuer.

The class of equity securities to which this Schedule 13D relates is the common stock, par value $0.001 per share (“Common Stock”), of Acceleron Pharma Inc., a Delaware corporation (“Acceleron”). The principal business address of Acceleron is 128 Sidney Street, Cambridge, Massachusetts 02139.

Item 2. Identity and Background.

This statement is filed by Celgene Corporation, a Delaware corporation (“Celgene”). The principal business address of Celgene is 86 Morris Avenue, Summit, New Jersey 07093. Celgene is a global biopharmaceutical company primarily engaged in the discovery, development and commercialization of innovative therapies designed to treat cancer and immune-inflammatory related diseases. Set forth on Schedule A hereto, which is incorporated herein by reference, is the name, business address, principal occupation or employment and the name, principal business and address of any corporation or other organization in which such employment is conducted and citizenship of each of Celgene’s directors and executive officers.

During the past five years, neither Celgene nor, to the knowledge of Celgene, any of the persons listed on Schedule A hereto, has been (i) convicted in a criminal proceeding (excluding traffic violations and similar misdemeanors), or (ii) a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

Item 3. Source and Amount of Funds or Other Consideration.

The information set forth or incorporated in Items 4 and 6 hereof are incorporated herein by reference.

The shares of Common Stock of Acceleron beneficially owned by Celgene were acquired with working capital of Celgene set aside for the general purpose of investing.

Item 4. Purpose of Transaction.

The information set forth or incorporated in Items 3 and 6 hereof are incorporated herein by reference.

On February 20, 2008, Celgene entered into a Collaboration, License and Option Agreement with Acceleron, and the parties amended the agreement as of August 2, 2011 (as amended, “ First Collaboration Agreement”), which is Exhibit 1 hereto. The purpose of the First Collaboration Agreement is for the parties to investigate and develop certain protein-based product candidates incorporating ActRIIA for the treatment, prevention, or modulation of diseases and conditions in humans. On August 2, 2011, Celgene and Acceleron entered into a second Collaboration, License and Option Agreement (“Second Collaboration Agreement” and, together with the First Collaboration Agreement, “Collaboration Agreements”), which is Exhibit 2 hereto. The purpose of the Second Collaboration Agreement is for the parties to investigate and develop ACE-536 in a defined field. Mr. George Golumbeski, who is currently Senior Vice President of Business Development of Celgene, is a member of the board of directors of Acceleron.

Concurrently with the First Collaboration Agreement, Celgene entered into a Series C-1 Convertible Preferred Stock Purchase Agreement (“Series C-1 Preferred Stock Purchase Agreement”) with Acceleron, pursuant to which Celgene agreed to purchase 1,831,502 shares of Acceleron’s Series C-1 Convertible Preferred Stock, par value $0.001 per share (“Series C-1 Preferred Stock”), for an aggregate purchase price of approximately $5.0 million. This transaction was consummated on March 24, 2008. This agreement is Exhibit 3 hereto. Under the Series C-1 Preferred Stock Purchase Agreement, Celgene committed to purchase up to $10.0 million of Common Stock in a private offering (“IPO Side-by-Side Purchase Commitment”) upon the closing of the sale of shares of Common Stock in an initial public offering (“Acceleron IPO”). The foregoing description of the IPO Side-by-Side Purchase Commitment is subject to, and qualified entirely by, the Series C-1 Preferred Stock Purchase Agreement.

On June 10, 2010, Celgene entered into a Series E Convertible Preferred Stock and Warrant Purchase Agreement with Acceleron and certain other investor parties named therein, pursuant to which Celgene agreed to purchase 145,985 shares of Acceleron’s Series E Convertible Preferred Stock, par value $0.001 per share (“Series E Preferred Stock”), and a warrant to purchase 155,916 shares of Common Stock (“Warrant”) for an aggregate purchase price of $458,392.90. This agreement is Exhibit 4 hereto. This transaction was consummated on or about July 9, 2010.

On December 22, 2011, Celgene entered into a Series F Convertible Preferred Stock Purchase Agreement (“Series F Preferred Stock Purchase Agreement”) with Acceleron and certain other investor parties named therein, pursuant to which Celgene agreed to purchase 7,961,784 shares of Acceleron’s Series F Convertible Preferred Stock, par value $0.001 per share (“Series F Preferred Stock”), for an aggregate purchase price of approximately $25.0 million. This agreement is Exhibit 5 hereto. This transaction was consummated on or about the date of the Series F Preferred Stock Purchase Agreement. In connection with the Series F Preferred Stock Purchase Agreement, Celgene also entered into an Amended and Restated Registration Rights Agreement (“Registration Rights Agreement”) with Acceleron and certain other investor parties named therein, pursuant to which Celgene and such other investor parties received certain registration rights to have the shares of Common Stock underlying certain securities of Acceleron, including the Series C-1 Preferred Stock, the Series E Preferred Stock, the Warrant and the Series F Preferred Stock registered under the Securities Act of 1933, as amended. The Registration Rights Agreement is Exhibit 6 hereto. Upon the request of certain investor parties, Acceleron agreed to prepare and file a registration statement with the Securities and Exchange Commission. The foregoing description of the registration rights is subject to, and qualified in its entirety by, the Registration Rights Agreement.

Effective September 5, 2013, Acceleron effected a 1-for-4 reverse split of its Common Stock and preferred stock, including the Series C-1 Preferred Stock, the Series E Preferred Stock, and the Series F Preferred Stock.

On September 24, 2013, pursuant to the IPO Side-by-Side Purchase Commitment under the Series C-1 Preferred Stock Purchase Agreement, Celgene entered into a Common Stock Purchase Agreement (“Common Stock Purchase Agreement”) with Acceleron, pursuant to which Celgene agreed to purchase approximately $10.0 million of Common Stock subject to and upon the closing of the Agios IPO. The Common Stock Purchase Agreement is Exhibit 7 hereto. Concurrently with the closing of the Acceleron IPO, this transaction was consummated on September 24, 2013.

Also on September 24, 2013, upon the closing of the Acceleron IPO, the shares of Series C-1 Preferred Stock, Series E Preferred Stock and Series F Preferred Stock owned by Celgene automatically converted into 2,506,220 shares of Common Stock. These shares of Series C-1 Preferred Stock, Series E Preferred Stock and Series F Preferred Stock were convertible into shares of Common Stock, and the Warrant was exercisable, at the election of Celgene prior to the Acceleron IPO.

Celgene acquired the shares of Common Stock reported on this Schedule 13D in furtherance of the purposes of the Collaboration Agreements.

Celgene intends to review its investment on a regular basis and, as a result thereof, may at any time or from time to time determine, either alone or as part of a group, (a) to acquire additional securities of Acceleron, through open market purchases, privately negotiated transactions or otherwise, (b) to dispose of all or a portion of the securities of Acceleron owned by it in the open market, in privately negotiated transactions or otherwise, or (c) to take any other available course of action, which could involve one or more of the types of transactions or have one or more of the results described in the next paragraph of this Item 4. Any such acquisition or disposition or other transaction would be made in compliance with all applicable laws and regulations. Notwithstanding anything contained herein, Celgene specifically reserves the right to change its intention with respect to any or all of such matters. In reaching any decision as to its course of action (as well as to the specific elements thereof), Celgene currently expects that it would take into consideration a variety of factors, including, but not limited to, the following: Acceleron’s business and prospects; other developments concerning Acceleron and its businesses generally; other business opportunities available to Celgene; changes in law and government regulations; general economic conditions; and money and stock market conditions, including the market price of the securities of Acceleron.

Except as set forth above, Celgene does not have any present plans which relate to or would result in:

(a) The acquisition by any person of additional securities of Acceleron, or the disposition of securities of Acceleron;

(b) An extraordinary corporate transaction, such as a merger, reorganization or liquidation, involving Acceleron or any of its subsidiaries;

(c) A sale or transfer of a material amount of assets of Acceleron or any of its subsidiaries;

(d) Any change in the present board of directors or management of Acceleron, including any plans or proposals to change the number or term of directors or to fill any existing vacancies on the board;

(e) Any material change in the present capitalization or dividend policy of Acceleron;

(f) Any other material change in Acceleron’s business or corporate structure;

(g) Changes in Acceleron’s charter, bylaws or instruments corresponding thereto or other actions which may impede the acquisition of control of Acceleron by any person;

(h) Causing a class of securities of Acceleron to be delisted from a national securities exchange or to cease to be authorized to be quoted in an inter-dealer quotation system of a registered national securities association;

(i) A class of equity securities of Acceleron becoming eligible for termination of registration pursuant to Section 12(g)(4) of the Securities Exchange Act of 1934, as amended; or

(j) Any action similar to any of those enumerated above.

Item 5. Interest in Securities of the Issuer.

(a), (b) and (c) — The information contained on the cover pages to this Schedule 13D and the information set forth or incorporated in Items 2, 3, 4 and 6 hereof are incorporated herein by reference.

As of September 24, 2013, following the consummation of the transactions contemplated by the Common Stock Purchase Agreement and the closing of the Acceleron IPO on such date, Celgene beneficially owns 3,211,866 shares of Common Stock (including 38,979 shares of Common Stock underlying the Warrant that is exercisable immediately), representing approximately 11.4% of the shares of Common Stock outstanding as of such date as determined is based on 28,086,834 shares of Common Stock outstanding as of September 24, 2013, which includes (i) 20,964,188 shares of Common Stock outstanding as of June 30, 2013, as reported by Acceleron in the Prospectus, (ii) 6,417,000 shares of Common Stock sold under the Prospectus (including 837,000 shares of Common Stock sold upon the exercise in full by the underwriters of their option to purchase additional shares to cover over-allotments), and (iii) 38,979 shares of Common Stock underlying the Warrant that is exercisable immediately).

(d) Not applicable.

(e) Not applicable.

Item 6. Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer.

The information set forth or incorporated in Items 4 hereof is incorporated herein by reference in response to this Item 6.

Mr. George Golumbeski, who is Senior Vice President of Business Development of Celgene, is a member of the board of directors of Acceleron. He is a party to an indemnification agreement with Acceleron, pursuant to which Acceleron may be required to indemnify Mr. Golumbeski for certain expenses, including attorneys’ fees, judgments, fines and settlement amounts, incurred by Mr. Golumbeski in any action or proceeding arising out of his service as one of Acceleron’s directors. A form of this agreement is Exhibit 8 hereto.

Other than as described in Items 3, 4 and 5, this Item 6, and the agreements set forth as exhibits hereto, to Celgene’s knowledge, there are no contracts, arrangements, understandings or relationships (legal or otherwise) among the persons named in Item 2 and between such persons and any person with respect to any securities of Acceleron.

Item 7. Material to Be Filed as Exhibits.

The following documents are filed as exhibits:

Exhibit 1 —	Collaboration, License and Option Agreement, dated as of February 20, 2008, as amended as of August 2, 2011, between Celgene and Acceleron (incorporated by reference to Exhibit 10.6 to the Registration Statement on Form S-1 filed by Acceleron on September 6, 2013)
Exhibit 2 —	Collaboration, License and Option Agreement, dated as of August 2, 2011, between Celgene and Acceleron (incorporated by reference to Exhibit 10.9 to the Registration Statement on Form S-1 filed by Acceleron on September 6, 2013)
Exhibit 3 —	Series C-1 Convertible Preferred Stock Purchase Agreement, dated as of February 20, 2008, between Celgene and Acceleron

Exhibit 4 —	Series E Convertible Preferred Stock and Warrant Purchase Agreement, dated as of June 10, 2010, among Celgene, Acceleron and other investor parties named therein
Exhibit 5 —	Series F Convertible Preferred Stock Purchase Agreement, dated as of December 22, 2011, among Celgene, Acceleron and other investor parties named therein
Exhibit 6 —	Amended and Restated Registration Rights Agreement, dated as of December 22, 2011, among Celgene, Acceleron and other investor parties named therein
Exhibit 7 —	Common Stock Purchase Agreement, dated as of September 24, 2013, between Celgene and Acceleron
Exhibit 8 —	Form of Indemnification Agreement between Acceleron and its directors and officers (incorporated by reference to Exhibit 10.1 to the Registration Statement on Form S-1 filed by Acceleron on August 7, 2013)

SIGNATURE

After reasonable inquiry and to the best of its knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

CELGENE CORPORATION
By:	/s/ Perry Karsen
	Perry Karsen
	Executive Vice President and Chief Operations Officer

EXHIBIT INDEX

Exhibit	Description
1	Collaboration, License and Option Agreement, dated as of February 20, 2008, as amended as of August 2, 2011, between Celgene and Acceleron (incorporated by reference to Exhibit 10.6 to the Registration Statement on Form S-1 filed by Acceleron on September 6, 2013)
2	Collaboration, License and Option Agreement, dated as of August 2, 2011, between Celgene and Acceleron (incorporated by reference to Exhibit 10.9 to the Registration Statement on Form S-1 filed by Acceleron on September 6, 2013)
3	Series C-1 Convertible Preferred Stock Purchase Agreement, dated as of February 20, 2008, between Celgene and Acceleron
4	Series E Convertible Preferred Stock and Warrant Purchase Agreement, dated as of June 10, 2010, among Celgene, Acceleron and other investor parties named therein
5	Series F Convertible Preferred Stock Purchase Agreement, dated as of December 22, 2011, among Celgene, Acceleron and other investor parties named therein
6	Amended and Restated Registration Rights Agreement, dated as of December 22, 2011, among Celgene, Acceleron and other investor parties named therein
7	Common Stock Purchase Agreement, dated as of September 24, 2013, between Celgene and Acceleron
8	Form of Indemnification Agreement between Acceleron and its directors and officers (incorporated by reference to Exhibit 10.1 to the Registration Statement on Form S-1 filed by Acceleron on August 7, 2013)

SCHEDULE A

Name, business address, present principal occupation or employment and place of citizenship of the directors and executive officers of Celgene Corporation

The name, business address and present principal occupation or employment of each of the directors and executive officers of Celgene Corporation are set forth below. The business address of each director and executive officer is c/o 86 Morris Avenue, Summit, New Jersey 07093. Unless otherwise indicated, each director and executive officer is a citizen of the United States.

CELGENE CORPORATION — BOARD OF DIRECTORS

Name And Position	Present Principal Occupation Or Employment
Robert J. Hugin Chairman of the Board, President and Chief Executive Officer	Chairman of the Board, President and Chief Executive Officer of Celgene Corporation
Richard W. Barker, D.Phil. Director (Citizen of the United Kingdom)	Director of the Centre for Accelerating Medical Innovations; a member of the Board of iCO Therapeutics, Inc.; Chairman of Stem Cells for Safer Medicine; Founder and Chairman of the Athenaeum Group; Senior Advisor of Aegate, Ltd.
Michael D. Casey Director	Formerly Chairman, President, Chief Executive Officer and a director of Matrix Pharmaceutical, Inc.
Carrie S. Cox Director	Chairman of the Board of Directors and Chief Executive Officer of Humacyte, Inc.
Rodman L. Drake Director	Formerly Managing Director of Baringo Capital, LLC
Michael A. Friedman, M.D. Director	Chief Executive Officer of City of Hope
Gilla Kaplan, Ph.D. Director	Head of the Laboratory of Mycobacterial Immunity and Pathogenesis at The Public Health Research Institute Center at the University of Medicine and Dentistry of New Jersey in Newark, New Jersey
James J. Loughlin Director	Formerly National Director of the Pharmaceuticals Practice at KPMG LLP
Ernest Mario, Ph.D. Director	Chief Executive Officer of Capnia, Inc.

CELGENE CORPORATION — EXECUTIVE OFFICERS

Name	Title
Robert J. Hugin	Chairman of the Board, President and Chief Executive Officer

Jacqualyn A. Fouse, Ph.D.	Executive Vice President and Chief Financial Officer
Mark. J. Alles	Executive Vice President and Global Head, Hematology and Oncology
Thomas O. Daniel, M.D.	Executive Vice President and President, Research and Early Development
Perry A. Karsen	Executive Vice President, Chief Operations Officer
Lawrence V. Stein	Executive Vice President, General Counsel and Corporate Secretary

EX-3

Exhibit 3

SERIES C-1 CONVERTIBLE PREFERRED STOCK

PURCHASE AGREEMENT

THIS SERIES C-1 CONVERTIBLE PREFERRED STOCK PURCHASE AGREEMENT (“Agreement”) is made as of February 20, 2008 (the “Execution Date”) by and among Acceleron Pharma Inc., a Delaware corporation (the “Corporation”) and Celgene Corporation, a Delaware corporation (the “Investor”).

WHEREAS, the Investor wishes to purchase from the Corporation, and the Corporation wishes to sell to the Investor, up to an aggregate of 1,831,502 shares of the Corporation’s Series C-1 Convertible Preferred Stock, $.001 par value per share (the “Series C-1 Preferred Stock”);

NOW, THEREFORE, in consideration of the premises and the mutual covenants and agreements herein contained, the parties hereby agree as follows:

SECTION 1. Amended and Restated Certificate of Incorporation. Prior to the SPA Closing (as defined in SECTION 3); the Corporation will file with the Secretary of State of the State of Delaware an Amended and Restated Certificate of Incorporation (the Certificate of Incorporation of the Corporation as so amended and restated and as in effect on the date hereof being hereinafter referred to as the “Certificate of Incorporation”), a copy of which is attached hereto as Exhibit 1.

SECTION 2. Purchase and Sale of the Series C-1 Preferred Stock. Subject to the terms and conditions of this Agreement, at the SPA Closing (as defined in SECTION 3), the Corporation agrees to issue and sell to the Investor 1,831,502 shares of Series C-1 Preferred Stock (the “Series C-1 Shares”) at the aggregate purchase price of $5,000,000.46. The Investor agrees to purchase from the Corporation at the SPA Closing (as defined in SECTION 3) the Series C-1 Shares at such aggregate purchase price.

SECTION 3. SPA Closing. The closing of the sale and purchase of the Series C-1 Shares (the “SPA Closing”) shall take place simultaneously with or as soon as practicable following the Closing, as defined in the License, Collaboration and Option Agreement between the Corporation and the Investor, dated the date hereof (the “License Agreement”), at the offices of Ropes & Gray LLP, One International Place, Boston, Massachusetts, or at such other location as may be agreed upon among the Investor and the Corporation. At the SPA Closing, the Corporation shall issue and deliver to the Investor a certificate or certificates for shares of Series C-1 Preferred Stock, registered in the name of such Investor, in the amount representing the Series C-1 Shares being purchased by such Investor at the SPA Closing, against payment by the Investor to the Corporation of the aggregate purchase price therefor in the form of (a) a certified or bank check payable to the order of the Corporation or (b) a wire transfer to a bank account designated by the Corporation or (c) any combination of (a) and (b).

SECTION 4. IPO Side-by-Side Purchase Commitment. Upon the consummation by Acceleron of its first underwritten public offering of its common stock, if such offering results in gross proceeds to the Corporation of not less than $35,000,000 (prior to underwriters’ discounts and commissions) (the “IPO”), Celgene will purchase (to the extent that Acceleron deems

permissible under the federal securities laws, the rules and regulations of the National Association of Securities Dealers, Inc., and all other applicable laws, rules and regulations), in a private offering concurrently with the IPO, at a purchase price equal to the price per share of common stock sold to the public in the IPO, a number of shares of Acceleron’s common stock equal to the Purchase Amount, as defined below, divided by the price per share of common stock sold to the public in the IPO. As used in this Section, the “Purchase Amount” shall be equal to $10,000,000, if the gross proceeds to the Corporation in the IPO is $50,000,000 or greater and twenty percent (20%) of the gross proceeds to the Corporation in the IPO if such gross proceeds are less than $50,000,000. Such purchase shall be on terms customary for such an investment. The obligations set forth in this SECTION 4 will terminate automatically upon the expiration or termination of the License Agreement.

SECTION 5. Representations and Warranties of the Corporation. The Corporation represents and warrants to the Investor as of the Execution Date and as of the SPA Closing as follows:

5.1 Organization. The Corporation is a corporation duly organized, validly existing and in good standing under the laws of the State of Delaware and has all requisite corporate power and authority to own and lease its properties, to carry on its business as presently conducted and as proposed to be conducted by it and to carry out the transactions contemplated by this Agreement. The Corporation is duly qualified as a foreign corporation and is in good standing in all such jurisdictions (which jurisdictions are listed in Schedule 5.1A) in which the conduct of its business or its ownership or leasing of property requires such qualification, except to the extent that any failure to be so qualified would not materially and adversely affect the financial condition, results of operations, assets, liabilities business or prospects of the Corporation.

5.2 Capitalization. Immediately prior to the SPA Closing, the entire authorized capital stock of the Corporation will consist of:

(a) 80,000,000 shares of Common Stock, $.001 par value per share (‘‘Common Stock”), of which (i) 8,067,083 shares have been issued and are outstanding, fully paid and nonassessable; (ii) no shares are held as treasury shares; (iii) 11,400,000 have been reserved for issuance upon exercise of options granted or to be granted under stock purchase, stock option or other equity incentive plans of the Corporation; (iv) 26,069,980 shares have been reserved for issuance upon conversion of the outstanding shares of Series A Preferred Stock of the Corporation and shares of Series A Preferred Stock issuable upon exercise of outstanding warrants; (v) 16,944,378 shares have been reserved for issuance upon conversion of the Series B Preferred Stock and shares of Series B Preferred Stock issuable upon exercise of outstanding warrants; (vi) 11,923,076 shares have been reserved for issuance upon conversion of the Series C Preferred Stock; and (vii) 1,831,502 shares have been reserved for issuance upon conversion of the Series C-1 Preferred Stock;

(b) 26,069,980 of Series A Preferred Stock, 25,643,980 shares of which are issued and outstanding;

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(c) 16,944,378 shares of Series B Preferred Stock, 16,816,810 of which are issued and outstanding;

(d) 11,923,077 shares of Series C Preferred Stock, 11,912,333 shares of which are issued and outstanding; and

(e) 1,831,502 shares of Series C-1 Preferred Stock, all of which are being issued at the SPA Closing and immediately thereafter will be issued and outstanding, fully paid and nonassessable and will be held by the Investor.

Schedule 5.2 contains a list of all holders of capital stock of the Corporation and options, warrants or rights to purchase such capital stock that are outstanding as of the Execution Date, in each case including the number of shares of capital stock held by, or subject to purchase pursuant to the exercise of any option, warrant or right held by, each such holder. Except as set forth in Schedule 5.2, there are no outstanding shares of capital stock of the Corporation or warrants, options, agreements, convertible securities or other commitments pursuant to which the Corporation is or may become obligated to issue any shares of its capital stock or other securities. Except as set forth in Schedule 5.2, the number of shares of capital stock, if any, issuable in connection with the securities described in the first sentence of this paragraph is not subject to adjustment by reason of the issuance of the Series C-1 Shares or the Reserved Shares (as defined in Section 5.22). There are no preemptive or similar rights to purchase or otherwise acquire shares of capital stock of the Corporation from the Corporation pursuant to any provision of law, the Certificate of Incorporation or the By-Laws or, except as set forth in Schedule 5.2, any agreement to which the Corporation is a party, or otherwise. Except as set forth in the Amended and Restated Registration Rights Agreement, Amended and Restated Voting Agreement, Amended and Restated Right of First Rehsal and Co-Sale Agreement and Amended and Restated Investor Rights Agreement (each as defined in Section 5.22) or in Schedule 5.2, there is no agreement, restriction or encumbrance with respect to the sale or voting of any shares of the Corporation’s capital stock (whether outstanding or issuable upon conversion or exercise of outstanding securities). The Corporation has not violated the Securities Act of 1933, as amended (the “Securities Act”) or any securities law of any state or other jurisdiction in connection with the issuance of any securities.

5.3 Equity Investments. The Corporation does not currently own, directly or indirectly, any capital stock or other proprietary interest in any corporation, association, trust, partnership, limited liability company, limited liability partnership, joint venture or other entity.

5.4 Financial Statements. Attached as Schedule 5.4 are (i) the unaudited consolidated balance sheet of the Corporation (the “Balance Sheet”) as of December 31, 2007 (the “Balance Sheet Date”) and (ii) the unaudited income statement and statement of cash flows of the Corporation for the twelve-month period ended the Balance Sheet Date (collectively with the Balance Sheet, the “Financial Statements”). The Financial Statements (a) are true and correct in all material respects, (b) are in accordance with the books and records of the Corporation, and (c) present fairly the financial position of the Corporation as of the Balance Sheet Date; each except that the Financial Statements may (x) be subject to normal year-end audit adjustments and (y) not contain all notes thereto which may be required in accordance with United States generally accepted accounting principles).

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5.5 Absence of Undisclosed Liabilities. Except as set forth in Schedule 5.5 or as reflected on the Balance Sheet, the Corporation has no liabilities of any nature (matured or unmatured, fixed or contingent) that individually or in the aggregate exceed $50,000.

5.6 Absence of Changes. Since the Balance Sheet Date there has not been (a) any change in the financial condition, results of operations, assets, liabilities, business or prospects of the Corporation that, individually or in the aggregate, is materially adverse, (b) any material asset or property of the Corporation made subject to a lien of any kind, except liens for taxes not yet due and payable, (c) any satisfaction or discharge of any lien, claim, or encumbrance or payment of any obligation by the Corporation, except in the ordinary course of business and that is not material to the financial condition, results of operations, assets, liabilities, business or prospects of the Corporation, (d) any waiver of any valuable right of the Corporation, or the cancellation of any debt or claim held by the Corporation, (e) any payment of dividends on, or other distribution with respect to, or any direct or indirect redemption or acquisition of, any shares of the capital stock of the Corporation, or any agreement or commitment therefor, (f) any mortgage, pledge, sale, assignment or transfer of any tangible or intangible assets of the Corporation, except in the ordinary course of business, (g) any loan by the Corporation to, or any loan to the Corporation from, any officer, director, employee or stockholder of the Corporation, or any agreement or commitment therefor, (h) any damage, destruction or loss (whether or not covered by insurance) materially and adversely affecting the assets, property or business of the Corporation, (i) any change in the accounting methods or practices followed by the Corporation, (j) any amendment to the Corporation’s Certificate of Incorporation or Bylaws, except as provided in this Agreement or (k) any agreement or commitment by the Corporation to do any of the things described in this Section 5.6. Each of the occurrences described in clauses (a) through (k) above is sometimes hereinafter referred to as a “Material Adverse Change.”

5.7 Encumbrances. The Corporation has good and marketable title to all of its property and assets, real, personal or mixed, tangible or intangible, free and clear of all liens, security interests, charges and other encumbrances of any kind, except (i) liens for taxes not yet due and payable, (ii) liens securing debt reflected on the Balance Sheet, and (iii) immaterial liens arising in the ordinary course of business.

5.8 Permits. The Corporation has all franchises, permits, licenses, authorizations, approvals, certificates, consents, orders and any similar authority (collectively, the “Permits”) from state, federal and regulatory authorities of the United States, including without limitation, the United States Food and Drug Administration (the “FDA”), necessary for the conduct of its business as now being conducted by it, except for those the lack of which would not reasonably be expected to cause a Material Adverse Change. All such Permits are valid and in full force and effect. There is no FDA enforcement action pending or, to the knowledge of the Corporation, threatened against the Corporation. The Corporation is not in default in any material respect under any of such franchises, permits, licenses or other similar authority.

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5.9 Intellectual Property Rights.

(a) All of the patents, trademarks, service marks, trade names, copyrights, applications for patents, trademarks, service marks and copyrights owned or licensed exclusively by the Corporation are listed in Schedule 5.9 (the “Scheduled Intellectual Property”);

(i) Except as set forth in Schedule 5.9, no royalties or other amounts are payable by the Corporation to other persons by reason of the ownership or use of the Scheduled Intellectual Property or the other Intellectual Property Rights (as defined below);

(ii) To the knowledge of the Corporation and except as set forth on Schedule 5.9, (i) no product or service marketed or sold by the Corporation and no Intellectual Property Rights licensed or proposed to be licensed by the Corporation as licensor violates any license or infringes any Intellectual Property Rights of another, and (ii) the Corporation has not received any notice that any of the Intellectual Property Rights identified in Schedule 5.9 or the operation of the business of the Corporation conflicts or will conflict with the Intellectual Property Rights of others; and

(b) There are no claims pending or threatened with respect to any Intellectual Property Rights necessary or required for the conduct of the business of the Corporation as currently conducted, nor, to the Corporation’s knowledge, does there exist any basis therefor.

As used herein, the term “Intellectual Property Rights” means all patents, trademarks, service marks, trade names, copyrights, inventions, trade secrets, know-how, proprietary processes and formulae, applications for patents, trademarks, service marks and copyrights, and other industrial and intellectual property rights.

5.10 Litigation. There is no action, suit, claim, proceeding or investigation, at law, in equity or otherwise, or by or before any governmental instrumentality or other agency, now pending or, to the knowledge of the Corporation, threatened against or affecting the Corporation, nor, to the knowledge of the Corporation, does there exist any basis therefor.

5.11 No Defaults. The Corporation is not in violation or breach of, or in default under, any provision of the Certificate of Incorporation or By-Laws, and there exists no condition, event or act which after notice, lapse of time, or both, may constitute a violation or breach of, or a default under, the Certificate of Incorporation or By-Laws. The Corporation is not in violation or breach of, or in default under any note, indenture, mortgage, lease, contract, purchase order or other instrument, document or agreement to which the Corporation is a party or by which it or any of its property is bound or affected or any ruling, writ, injunction, order, judgment or decree of any court, administrative agency or other governmental body, except to the extent that any of the foregoing individually or in the aggregate would not materially and adversely affect the financial condition, results of operations, assets, liabilities business or prospects of the Corporation. To the knowledge of the Corporation, there exists no condition, event or act which after notice, lapse of time, or both, may constitute a violation or breach of, or a default under, any of the foregoing.

5.12 Employment of Founders, Officers, Employees and Consultants. To the knowledge of the Corporation and except as set forth on Schedule 5.12, no employee of the Corporation is in violation of any term of any contract or covenant (either with the Corporation

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or with another entity) relating to employment, consulting, assignment of inventions, proprietary information disclosure, non-competition or non-solicitation. To the knowledge of the Corporation and except as set forth on Schedule 5.12, neither of Jasbir Seehra or John Knopf (the “Founders”) is obligated under any contract or subject to any judgment, decree or administrative order that would conflict or interfere with (i) the performance of such Founder’s duties as an employee, consultant, director or officer of the Corporation, as the case may be, or (ii) the Corporation’s business as conducted or proposed to be conducted.

5.13 Taxes. The Corporation has filed all federal, state, local and foreign tax returns which are required to be filed by it and all such returns are true and correct in all material respects. The Corporation has paid all taxes shown as due thereon, or pursuant to any assessments received by it or which it is obligated to withhold from amounts owing to any employee, creditor or third party, except, in each case, for those taxes which are not yet due and payable pursuant to such returns.

5.14 Material Agreements. Except as set forth in Schedule 5.14, the Corporation is not a party to any written or oral (a) contract with any labor union; (b) contract for the future purchase of fixed assets or materials, supplies or equipment in excess of normal operating requirements; (c) contract for the employment of any officer, employee or other person or any contract with any person on a consulting basis; (d) bonus, pension, profit-sharing, retirement, stock purchase, stock option, hospitalization, medical insurance or similar plan, contract or understanding in effect with respect to employees or any of them or the employees of others; (e) agreement or indenture relating to the borrowing of money or to the mortgaging, pledging or otherwise placing a lien on any assets of the Corporation; (f) guaranty of any obligation for borrowed money or otherwise; (g) lease or agreement under which the Corporation is lessee of or holds or operates any property, real or personal, owned by any other party; (h) lease or agreement under which the Corporation is lessor of or permits any third party to hold or operate any property, real or personal, owned or controlled by the Corporation; (i) license or lease agreement with respect to any Intellectual Property Rights; (j) agreement or other commitment for capital expenditures in excess of $50,000; (k) contract, agreement or commitment under which the Corporation is obligated to pay any broker’s fees, finder’s fees or any such similar fees, to any third party; or (l) any other contract, agreement, arrangement or understanding which is material to the business of the Corporation. Each contract, agreement, arrangement, understanding or commitment listed on Schedule 5.14 (each a “Material Agreement”) is a valid and binding obligation of the Corporation, and, to the knowledge of the Corporation, each other party thereto, enforceable against the Corporation in accordance with its terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)), and has been duly executed and delivered by the Corporation and, to the knowledge of the Corporation, each other party thereto. With respect to each Material Agreement, (a) the Corporation and, to the knowledge of the Corporation, each other party to such agreement has performed in all material respects all obligations required to be performed to date under such Material Agreement; (b) to the knowledge of the Corporation, no party to such Material Agreement is in default or arrears under the terms of such Material Agreement; and (c) to the knowledge of the Corporation, no condition exists or event has occurred that, with the giving of notice or lapse of time or both, would constitute a default under

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such Material Agreement. The Corporation has furnished to the Investor true and correct copies of all such written agreements and other documents, in each case to the extent requested by the Investor or its authorized representatives.

5.15 ERISA. Neither the Corporation nor any entity required to be aggregated with the Corporation under Sections 414(b), (c), (m) or (n) of the Internal Revenue Code of 1986, as amended (the “Code”), sponsors, maintains, has any obligation to contribute to, has any liability under, or is otherwise a party to, any Benefit Plan. For purposes of this Agreement, “Benefit Plan” shall mean any plan, fund, program, policy, arrangement or contract, whether formal or informal, which is in the nature of (i) an employee pension benefit plan (as defined in Section 3(2) of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”)) or (ii) an employee welfare benefit plan (as defined in section 3(1) of ERISA).

5.16 U.S. Real Property Holding Corporation. The Corporation is not now and has never been a “United States real property holding corporation,’’ as defined in Section 897(c)(2) of the Code and Section 1.897-2(b) of the Treasury Regulations promulgated by the Internal Revenue Service, and the Corporation has never filed with the Internal Revenue Service a statement with its United States income tax returns under Section 1.897-2(h) of such Regulations stating that any shares of its capital stock constitute a U.S. real property interest within the meaning of Section 897(c)(1) of the Code.

5.17 Environmental Protection. The Corporation has not caused or allowed, or contracted with any party for, the generation, use, transportation, treatment, storage or disposal of any Hazardous Substances (as defined below) in connection with the operation of its business or otherwise. The Corporation does not presently own or lease any real property. The operation of the business of the Corporation is in compliance with all applicable Environmental Laws (as defined below) and orders or directives of any governmental authorities having jurisdiction under such Environmental Laws, including, without limitation, any Environmental Laws or orders or directives with respect to any cleanup or remediation of any release or threat of release of Hazardous Substances. The Corporation has not received any citation, directive, letter or other communication, written or oral, or any notice of any proceeding, claim or lawsuit, from any person arising out of the conduct of its operations, and the Corporation is not aware of any basis therefor. The Corporation has obtained and is maintaining in full force and effect all necessary permits, licenses and approvals required by all Environmental Laws applicable to the business operations conducted by the Corporation, and is in compliance with all such permits, licenses and approvals. For the purposes of this Agreement, the term “Environmental Laws” shall mean any Federal, state or local law or ordinance or regulation pertaining to the protection of human health or the environment, including, without limitation, the Comprehensive Environmental Response, Compensation, and Liability Act, 42 U.S.C. Sections 9601, et seq., the Emergency Planning and Community Right-to-Know Act, 42 U.S.C. Sections 11001, et seq., and the Resource Conservation and Recovery Act, 42 U.S.C. Sections 6901 et seq. For purposes of this Agreement, the term “Hazardous Substances” shall include oil and petroleum products, asbestos, polychlorinated biphenyls, urea formaldehyde and other materials classified as hazardous or toxic under any Environmental Laws.

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5.18 Foreign Corrupt Practices Act. The Corporation has not taken any action which would cause it to be in violation of the Foreign Corrupt Practices Act of 1977, as amended, or any rules and regulations thereunder. There is not now, and there has never been, any employment by the Corporation of, or beneficial ownership in the Corporation by, any governmental or political official in any country in the world.

5.19 Federal Reserve Regulations. The Corporation is not engaged in the business of extending credit for the purpose of purchasing or carrying margin securities (within the meaning of Regulation U of the Board of Governors of the Federal Reserve System), and no part of the proceeds of the sale of Series C-1 Shares will be used to purchase or carry any margin security or to extend credit to others for the purpose of purchasing or carrying any margin security or in any other manner which would involve a violation of any of the regulations of the Board of Governors of the Federal Reserve System.

5.20 Compliance. The Corporation has complied in all material respects with all federal, state, local and foreign laws applicable to its business. The Corporation has all federal, state, local and foreign governmental licenses, registrations and permits necessary for the conduct of its business, except for those the absence of which would not materially and adversely affect the financial condition, results of operations, assets, liabilities business or prospects of the Corporation; such licenses, registrations and permits are in full force and effect; and there have been no violations of any such licenses, registrations or permits except for violations that individually and in the aggregate would not have a material adverse effect on the financial condition, results of operations, assets, liabilities, business or prospects of the Corporation. No proceeding is pending, nor has the Corporation received any oral or written notice of any proceeding that is threatened, to revoke or limit any of the foregoing licenses, registrations or permits. Without limiting the generality of the foregoing, the Corporation has complied in all material respects with all applicable laws relating to the employment of labor, including provisions relating to wages, hours, equal opportunity, collective bargaining and the payment of Social Security and other taxes, and with ERISA.

5.21 Insurance. Schedule 5.21 sets forth each insurance policy (specifying the insurer, the amount of coverage, the type of insurance, the policy number, the expiration date, the annual premium and any pending claims thereunder) maintained by the Corporation on its properties, assets, business or personnel. No notice from any insurance carrier has been received by the Corporation claiming that the Corporation is in default with respect to any provision contained in any insurance policy listed on Schedule 5.21.

5.22 Authorization of Transaction Documents. The execution, delivery and performance by the Corporation of (a) this Agreement, (b) the Amended and Restated Registration Rights Agreement to be dated as of the date of the SPA Closing, by and among the Corporation and the Investor and certain other parties in the form of Exhibit 5.22A (the “Amended and Restated Registration Rights Agreement”), (c) the Amended and Restated Voting Agreement to be dated as of the date of the SPA Closing, by and among the Corporation and the other parties thereto in the form of Exhibit 5.22B (the “Amended and Restated Voting Agreement”), (d) the Amended and Restated Right of First Refusal and Co-Sale Agreement to be dated as of the date of the SPA Closing, by and among the Corporation and the other parties thereto in the form of Exhibit 5.22C (the “Amended and Restated Right of First Refusal and Co-Sale Agreement”), and (e) the Amended and Restated Investor Rights Agreement to be dated as of the date of the SPA Closing, by and among the Corporation and the other parties thereto in the

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form of Exhibit 5.22D (the “Amended and Restated Investor Rights Agreement” and, together with this Agreement, the Amended and Restated Registration Rights Agreement, the Amended and Restated Voting Agreement, the Amended and Restated Right of First Refusal and Co-Sale Agreement and the Amended and Restated Investor Rights Agreement, the “Transaction Documents”) have been (or will be, with respect to those agreements to be dated the date of the SPA Closing) duly authorized by all requisite corporate action. The Corporation has duly authorized and has (or will, with respect to those agreements to be dated the date of the SPA Closing, on or prior to the SPA Closing), executed and delivered each Transaction Document, and each Transaction Document constitutes (or will constitute, with respect to those agreements to be dated the date of the SPA Closing) the valid and binding obligation of the Corporation, enforceable in accordance with its terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)). The execution, delivery and performance of the Transaction Documents, the issuance, sale and delivery of the Series C-1 Shares and the shares of Common Stock issuable upon conversion of the Series C-1 Shares (the “Reserved Shares”), and compliance with the provisions hereof and thereof by the Corporation do not and will not, with or without the passage of time or the giving of notice or both, violate, conflict with or result in any breach of any of the terms, conditions or provisions of, or constitute a default (or give rise to any right of termination, cancellation or acceleration) under, or result in the creation of any lien, security interest, charge or encumbrance upon any of the properties or assets of the Corporation under, the Certificate of Incorporation or Bylaws, any Material Agreement, or any provision of law, statute, rule or regulation or any ruling, writ, injunction, order, judgment or decree of any court naming the Corporation, or administrative agency or other governmental body.

5.23 Authorization of Series C-1 Shares and Reserved Shares. The issuance, sale and delivery hereunder by the Corporation of the Series C-1 Shares have been duly authorized by all requisite corporate action of the Corporation, and when so issued, sold and delivered the Series C-1 Shares will be validly issued and outstanding, fully paid and nonassessable, and not subject to preemptive or any other similar rights of the stockholders of the Corporation or others. The issuance and delivery of the Reserved Shares have been duly authorized by all requisite corporate action of the Corporation, and the Reserved Shares shall have been duly reserved for issuance upon conversion of any or all of the Series C-1 Shares, and when so issued and delivered upon conversion of the Series C-1 Shares, the Reserved Shares will be validly issued and outstanding, fully paid and nonassessable, and not subject to preemptive or any other similar rights of the stockholders of the Corporation or others.

5.24 Related Transactions. Except as set forth in Schedule 5.24, no director, officer or employee of the Corporation nor any “associate” (as defined in Rule 12b-2 promulgated under the Securities Exchange Act of 1934, as amended (the “Exchange Act”)) of any such person is indebted to the Corporation, nor is the Corporation indebted (or committed to make loans or extend or guarantee credit) to any such person, nor is any such person a party to any transaction (other than as an employee or consultant) with the Corporation providing for the furnishing of services by, or rental of real or personal property from, or otherwise requiring cash payments to, any such person.

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5.25 Offerees. During the six months prior to the date of this Agreement, the Corporation has not, either directly or through any agent, offered any Common Stock, Series C-1 Preferred Stock or other securities convertible into Common Stock, Series C-1 Preferred Stock or any security or securities similar to any thereof, for sale to, or solicited any offers to buy any Common Stock, Series C-1 Preferred Stock or other securities convertible into Common Stock, Series C-1 Preferred Stock or any such similar security or securities from, or otherwise approached or negotiated in respect thereof with, any person or entity other than the Investor and a limited number of institutional or sophisticated investors.

5.26 No Governmental Consent or Approval Required. No authorization, consent, approval or other order of, declaration to, or filing with, any governmental agency or body is required to be made or obtained by the Corporation for or in connection with the valid and lawful authorization, execution and delivery by the Corporation of the Transaction Documents, for or in connection with the valid and lawful authorization, issuance, sale and delivery of the Series C-1 Shares or for or in connection with the valid and lawful authorization, reservation, issuance, sale and delivery of the Reserved Shares, except the exemptive filings under applicable securities laws set forth on Schedule 5.26, which are not required to be made until after the SPA Closing and which shall be made on a timely basis.

5.27 Registration Rights. Except as contemplated by the Amended and Restated Registration Rights Agreement, no person has any right to cause the Corporation to effect the registration under the Securities Act of any shares of Common Stock or any other securities of the Corporation.

5.28 Employees. Except as set forth on Schedule 5.28, each employee or consultant now employed or engaged by the Corporation who has access to confidential or proprietary information of the Corporation has executed an agreement with the Corporation relating to, among other matters, non-disclosure of confidential and proprietary information and the assignment of intellectual property, each of which agreements is listed on Schedule 5.28 and is in full force and effect. The Corporation has complied in all material respects with all applicable state and federal equal opportunity, minimum wage, immigration, workforce reduction and other laws related to employment and termination of employment, except in those cases where such non-compliance would not reasonably be expected to cause a Material Adverse Change. No officer or key employee of the Corporation has advised the Corporation (orally or in writing) that he intends to terminate employment with the Corporation.

5.29 Exemptions from Securities Laws. Subject to the accuracy of the representations and warranties of the Investor set forth in SECTION 6 hereof, the provisions of Section 5 of the Securities Act are inapplicable to the offering, issuance, sale and delivery of the Series C-1 Shares and the Reserved Shares, and no consent, approval, qualification or registration or filing under any state securities laws is required in connection therewith, except the exemptive filings set forth on Schedule 5.26, which are not required to be made until after the SPA Closing and which shall be made on a timely basis.

5.30 Investment Company Act. The Corporation is not an “investment company,” or a company “controlled” by an “investment company,” within the meaning of the Investment Company Act of 1940, as amended.

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5.31 83(b) Elections. To the Corporation’s knowledge, all individuals who have purchased unvested shares of the Corporation’s Common Stock have timely filed elections under Section 83(b) of the Code.

5.32 Disclosure. Neither this Agreement nor any other certificate, document or written statement furnished to the Investor by or on behalf of the Corporation contains any untrue statement of a material fact or omits to state a material fact necessary in order to make the statements contained herein or therein not misleading.

SECTION 6. Representations and Warranties of the Investor. The Investor represents and warrants to the Corporation as of the Execution Date and as of the SPA Closing as follows:

6.1 Purchase for Investment. The Investor is acquiring the Series C-1 Shares purchasable by it hereunder for its own account, for investment and not for, with a view to, or in connection with, any distribution or public offering thereof within the meaning of the Securities Act.

6.2 Unregistered Securities; Legend. The Investor understands that the Series C-1 Shares and the Reserved Shares have not been, and will not be, registered under the Securities Act or any state securities law, by reason of their issuance in a transaction exempt from the registration requirements of the Securities Act and such laws, that the Series C-1 Shares and the Reserved Shares must be held indefinitely unless they are subsequently registered under the Securities Act and such laws or a subsequent disposition thereof is exempt from registration, that the certificates for the Series C-1 Shares and the Reserved Shares shall bear a legend to such effect, and that appropriate stop transfer instructions may be issued. Such Investor further understands that such exemption depends upon, among other things, the bona fide nature of such Investor’s investment intent expressed herein.

6.3 Status of the Investor. The Investor has not been formed for the specific purpose of acquiring the Series C-1 Shares pursuant to this Agreement. The Investor understands the term “accredited investor” as used in Regulation D promulgated under the Securities Act and represents and warrants to the Corporation that the Investor is an “accredited investor” for purposes of acquiring the Series C-1 Shares purchasable by it hereunder.

6.4 Knowledge and Experience; Economic Risk. The Investor has sufficient knowledge and experience in business and financial matters and with respect to investment in securities of privately held companies so as to enable it to analyze and evaluate the merits and risks of the investment contemplated hereby and is capable of protecting its interest in connection with this transaction. The Investor is able to bear the economic risk of such investment, including a complete loss of the investment.

6.5 Access to Information. The Investor acknowledges that the Investor and its representatives have had the opportunity to ask questions and receive answers from officers and representatives of the Corporation concerning the Corporation and its business and the transactions contemplated by this Agreement and to obtain any additional information which the Corporation possesses or can acquire that is necessary to verify the accuracy of the information regarding the Corporation herein set forth or otherwise desired in connection with its purchase of the Series C-1 Shares purchasable by it hereunder.

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6.6 Rule 144. The Investor understands that the exemption from registration afforded by Rule 144 (the provisions of which are known to such Investor) promulgated by the Securities and Exchange Commission under the Securities Act depends upon the satisfaction of various conditions, and that such exemption is not currently available.

6.7 Place of Business. The Investor has listed its principal place of business or registered address next to its name on the signature page hereto.

6.8 Authorization of Transaction Documents. The Investor has duly authorized and has (or will, with respect to those agreements to be dated the date of the SPA Closing, on or prior to the SPA Closing) executed and delivered the Transaction Documents to which the Investor is a party, and such Transaction Documents constitute (or will constitute, with respect to those agreements to be dated the date of the SPA Closing) the valid and binding obligation of the Investor, enforceable against the Investor in accordance with their terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)).

SECTION 7. Conditions Precedent to SPA Closing by the Investor. The obligation of the Investor to purchase and pay for the Series C-1 Shares being purchased by the Investor at the SPA Closing is subject to satisfaction (or waiver by the Investor) of the following conditions precedent at or before the SPA Closing:

(a) Corporate Proceedings. All corporate and other proceedings to be taken and all waivers and consents to be obtained in connection with the transactions contemplated by this Agreement shall have been taken or obtained and all documents incident to such transactions shall be reasonably satisfactory in form and substance to the Investor and its counsel, who shall have received all such originals or certified or other copies of such documents as they may reasonably request.

(b) Representations and Warranties Correct. The representations and warranties made by the Corporation in SECTION 5 hereof (each as modified by any schedule referred to therein, but disregarding any updates to such schedules after the date hereof) shall be true and correct when made, and shall be true and correct in all material respects at the time of the SPA Closing with the same force and effect as if they had been made at and as of the time of the SPA Closing.

(c) Compliance with Covenants. The Corporation shall have duly complied with and performed all covenants and agreements of the Corporation herein which are required to be complied with and performed at or before the SPA Closing.

(d) Certificate of Compliance. The Corporation shall have provided to, the Investor a certificate, dated the date of the SPA Closing, in form and substance reasonably satisfactory to the Investor, confirming that (i) the representations and warranties made by the

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Corporation in SECTION 5 hereof (each as modified by any schedule referred to therein, including any updates to such schedules prior to the SPA Closing) were true and correct as of the date hereof and are true and correct in all material respects as of the SPA Closing, and (ii) the Corporation has duly complied with and performed all covenants and agreements of the Corporation herein which are required to be complied with and performed at or before the SPA Closing.

(e) Secretary’s Certificate. The Secretary or an Assistant Secretary of the Corporation shall have delivered to the Investor a certificate, dated the date of the SPA Closing, certifying: (a) that attached thereto is a true and complete copy of the By-Laws of the Corporation as in effect on the date of such certification; (b) that attached thereto is a true and complete copy of all resolutions adopted by the Board of Directors and the stockholders of the Corporation authorizing the transactions contemplated by this Agreement and any other actions necessary thereto; (c) that attached thereto is a true and complete copy of the Certificate of Incorporation as in effect on the date of such certification; and (d) to the incumbency and signatures of the officers of the Corporation executing the Transaction Documents.

(f) Opinion of Counsel. The Investor shall have received an opinion of Ropes & Gray LLP, counsel for the Corporation, addressed to the Investor, in substantially the form attached hereto as Exhibit 7(f).

(g) Related Agreements and Documents. At or before the SPA Closing, the parties thereto shall have executed and delivered the Transaction Documents, and the Corporation shall have delivered to the Investor such other documents consistent with the terms hereof as they shall reasonably request.

(h) Securities Matters. All consents, approvals, qualifications, registrations, notices and filings required to be obtained or effected as of the SPA Closing under any applicable securities laws of any state or other jurisdiction in connection with the issuance, sale and delivery of the Series C-1 Shares and the Reserved Shares shall have been obtained or ‘ effected and copies of the same delivered to the Investor.

(i) Delivery of Certificates for Series C-1 Shares. The Corporation shall have delivered to the Investor a certificate for the Series C-1 Shares being purchased by the Investor at the SPA Closing, registered in the name of the Investor.

SECTION 8. Conditions Precedent to SPA Closing by the Corporation. The obligation of the Corporation to issue and sell the Series C-1 Shares being sold to the Investor at the SPA Closing is subject to satisfaction (or the waiver by the Corporation) of the following conditions precedent at or before the SPA Closing:

8.1 Representations and Warranties. The representations and warranties made in SECTION 6 hereof by the Investor purchasing shares at the SPA Closing shall be true and correct when made, and shall be true and correct in all material respects at the time of the SPA Closing with the same force and effect as if they had been made at and as of the time of the SPA Closing.

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8.2 Related Agreements and Documents. At or before the SPA Closing, the parties thereto shall have executed and delivered the Transaction Documents.

8.3 Tender of Payment. At such SPA Closing, the Investor shall have tendered payment for the Series C-1 Shares being sold at the SPA Closing.

SECTION 9. Fees; Brokers.

9.1 Fees of Counsel. Each Party shall pay its own fees and expenses in connection with the SPA Closing.

9.2 Brokers. The Corporation represents and warrants to the Investor that (a) neither the Corporation nor any of its officers, directors, employees or stockholders, has employed any broker or finder in connection with the transactions contemplated by this Agreement, and (b) no person or entity will have, as a result of the transactions contemplated by this Agreement, any right to, interest in, or claim against or upon the Corporation or the Investor for, any commission, fee or other compensation as a finder or broker because of any act or omission by the Corporation or any agent of the Corporation.

SECTION 10. Remedies. In case any one or more of the representations, warranties, covenants or agreements set forth in this Agreement shall have been breached by the Corporation or any Founder, the Investor may proceed to protect and enforce their rights either by suit in equity or by action at law, including, but not limited to, an action for damages as a result of any such breach or an action for specific performance of any such covenant or agreement contained in this Agreement.

SECTION 11. Indemnification; Limitations on Liability.

11.1 The Corporation shall indemnify, defend and hold the Investor harmless from and against all liabilities, losses, and damages, together with all reasonable costs and expenses related thereto (including, without limitation, reasonable legal and accounting fees and expenses), which would not have been incurred if (a) all of the representations and warranties of the Corporation in the Transaction Documents (each as modified by any schedule referred to therein, including any updates to such schedules prior to the SPA Closing) had been true and correct when made and at the time of the SPA Closing, and (b) all of the covenants and agreements of the Corporation in the Transaction Documents had been duly and timely complied with and performed; provided, however, that the aggregate liability of the Corporation to the Investor under this Section 11.1 shall not exceed the aggregate purchase price of the Series C-1 Shares purchased by such Investor hereunder.

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SECTION 12. Exchanges; Lost, Stolen or Mutilated Certificates. Upon surrender by Investor to the Corporation of any certificate representing Series C-1 Shares or Reserved Shares, the Corporation at its expense shall issue in exchange therefor, and deliver to the Investor, new certificates representing such Series C-1 Shares or Reserved Shares, as the case may be, in such amounts or denominations as may be requested by the Investor. Upon receipt of evidence satisfactory to the Corporation of the loss, theft, destruction or mutilation of any certificate representing any Series C-1 Shares or Reserved Shares and in case of any such loss, theft or destruction, upon delivery of an indemnity agreement satisfactory to the Corporation, or in case of any such mutilation, upon surrender and cancellation of such certificate, the Corporation at its expense shall issue and deliver to such Investor a new certificate for such Series C-1 Shares or Reserved Shares, of like tenor, in lieu of such lost, stolen or mutilated certificate.

SECTION 13. Standstill Agreement. In the event that the SPA Closing has occurred, except as provided in SECTION 4, neither the Investor nor any of its Representatives (as defined below) (collectively the “Investor Related Parties”) will, in any manner, directly or indirectly, at any time prior to the one-year anniversary of the consummation of the Corporation’s, or any Affiliate (as defined below) of the Corporation’s initial public offering

13.1 make, effect, initiate, directly participate in or cause any acquisition of beneficial ownership of any securities of the Corporation or any securities of any Affiliate of the Corporation; or

13.2 enter into any discussions, negotiations, arrangement or agreement with any person other than the Corporation relating to any of the foregoing; or

13.3 request or propose, publicly or to shareholders (except in a shareholder’s capacity as Chief Executive Officer of the Corporation) of the Corporation or its Affiliates, that the Corporation or any of the Corporation’s Representatives (as defined below) amend, waive or consider the amendment or waiver of any provision set forth in this SECTION 13.

Notwithstanding the foregoing, the provisions of this SECTION 13 shall not apply to (i) the exercise by any of the Investor Related Parties of any rights available under the Transaction Documents or to shareholders generally pursuant to any transaction described in Section 13.1 above, provided that such Investor Related Party has not then either directly or as a member of a Group made, effected, initiated or caused such transaction to occur, (ii) any activity by any of the Investor Related Parties after the Corporation or any other third party unrelated to any of the Investor Related Parties has made any public announcement of its intent to solicit or engage in any transaction regarding the acquisition of beneficial ownership of greater than fifty percent (50%) of the securities of the Corporation or any securities of any Affiliate of the Corporation, (iii) the purchase of beneficial ownership of any securities of the Corporation or any securities of any Affiliate of the Corporation to maintain such Investor Related Parties’ proportionate holdings of voting securities or other ownership interests in the Corporation or such Affiliate following the issuance of securities by the Corporation or such Affiliate taking place after the consummation of such Person’s initial public offering, (iv) if any third party biotechnology or pharmaceutical company has acquired beneficial ownership of any securities of the Corporation or any securities of any Affiliate of the Corporation, the acquisition of beneficial ownership of any securities of the Corporation or such Affiliate such that the Investor Related Parties’

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ownership of voting securities or other ownership interests in such Person will be no less than the holdings of voting securities or other ownership interests of such third party biotechnology or pharmaceutical company, or (v) the acquisition of an aggregate of more than fifty percent (50%) of the outstanding voting securities or other ownership interests of the Corporation or any Affiliate of the Corporation in a single transaction; provided however, with respect to subpart (ii), if any of the Investor Related Parties or such third party terminates or announces its intent to terminate such transaction and such Investor Related Party (A) has not previously made any public announcement of its intent to solicit or engage in any transaction of the type referred to in this Section 13.1 above, or (B) in the event that such public announcement has been made by any of the Investor Related Parties, such Investor Related Party has terminated or announced its intent to terminate such transaction, then the provisions of this SECTION 13 shall again be applicable.

For purposes of this Section, “Representatives” of a party will be deemed to include each person or entity that is or becomes (i) an Affiliate of such party, or (ii) an officer, director, employee, partner, attorney, advisor, accountant, agent or representative of such party or of any of such party’s Affiliates, provided such person is acting on behalf of such party or such party’s Affiliate. “Group” means two or more Persons acting as a partnership, limited partnership, syndicate or other group for the purpose of acquiring, holding or disposing of securities of the Corporation. “Affiliate(s)” means, with respect to a Person, any Person that controls, is controlled by, or is under common control with such first Person. For purposes of this definition only, “control” means (a) to possess, directly or indirectly, the power to direct the management or policies of a Person, whether through ownership of voting securities or by contract relating to voting rights or corporate governance, or (b) to own, directly or indirectly, more than fifty percent (50%) of the outstanding voting securities or other ownership interests of such Person. “Person” means any individual, corporation, partnership, trust, limited liability company, association or other entity.

SECTION 14. Confidentiality.

14.1 For purposes of this SECTION 14:

(a) “Confidential Information” means (i) Investment Information and (ii) Scientific Information.

(b) “Investment Information” means any and all non-public scientific, technical, business or financial information, in whatever form (written, oral or visual) relating to the Corporation and delivered to the Investor other than Scientific Information. For clarity, “Investment Information” includes any such information relating specifically to programs based on ActRIIB, such as ACE-031, and programs based on ALK-1, such as ACE-041, disclosed by the Corporation to the Investor after December 20, 2007.

(c) “Purpose” means the exercise of Investor’s rights and the performance of Investor’s obligations pursuant to the License Agreement.

(d) “Scientific Information” means any and all non-public scientific, technical, business or financial information, in whatever form (written, oral or visual) relating specifically to programs based on ActRIIA, such as ACE-011, programs based on

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ActRIIE3, such as ACE-031, programs based on ALK-1, such as ACE-041, programs based on antibodies that bind to Activin A and/or Activin B, and programs based on antibodies that bind to BMP3, disclosed by the Corporation to the Investor provided that such information relating specifically to programs based on ActRIIB, such as ACE-03 1 and programs based on ALK-1, such as ACE-041 disclosed by the Corporation to the Investor after December 20,2007 shall not be considered Scientific Information hereunder.

14.2 Investor covenants that it will (a) hold in confidence all Confidential Information and not publish or disclose it except as provided hereunder; (b) use the Scientific Information solely for the Purpose and the Investment Purpose (defined below); (c) treat Confidential Information with the same degree of care it uses to protect its own confidential information but in no event with less than a reasonable degree of care; (d) reproduce the Scientific Information solely to accomplish the Purpose and the Investment Purpose, and (e) disclose Confidential Information solely to its employees or consultants on a need-to-know basis, provided that each such employee and consultant is bound by obligations of confidentiality at least as restrictive as those set forth in this Agreement.

14.3 In addition to all obligations of Investor pursuant to Section 14.2, Investor also covenants that it will (a) use the Investment Information solely for the purpose of evaluating its equity investment in the Corporation (the “Investment Purpose”); (b) reproduce the Investment Information only as necessary and solely to accomplish the Investment Purpose, and (c) disclose Investment Information solely to individuals that are employees within Investor’s Business Development, Finance or Legal Departments, and outside counsel retained by employees within Investor’s Business Development, Finance or Legal Departments, in each case solely to the extent that such individuals have a need to use the Investment Information for the Investment Purpose and provided that each such employee and consultant is bound by obligations of confidentiality at least as restrictive as those set forth in this Agreement.

14.4 Investor will have no obligations of confidentiality and non-use with respect to any portion of the Confidential Information which:

(a) is or later becomes generally available to the public by use, publication or the like, through no fault of Investor;

(b) is obtained from a third party who had the legal right to disclose the same to Investor;

(c) Investor already possesses, as evidenced by its written records that predate the receipt thereof, or

(d) is hereafter independently developed by Investor without reference to the information provided by the Corporation.

Investor may disclose Confidential Information as required by law or applicable regulation including, without limitation, by oral questions, interrogatories, request for information or documents, subpoena, civil investigative demand or other similar process. Investor will give the Corporation prompt notice thereof so that the Corporation may attempt to seek an appropriate protective order.

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14.5 It is understood and agreed that the Corporation may be irreparably injured by a breach of this SECTION 14; that money damages would not be an adequate remedy for any such breach; and that the Corporation will be entitled to seek equitable relief, including injunctive relief and specific performance, as a remedy for any such breach, and such remedy will not be the Corporation’s exclusive remedy for any breach of this SECTION 14.

14.6 Investor acknowledges that Confidential Information may still be under development, or may be incomplete, and that such information may relate to products that are under development or are planned for development. THE CORPORATION MAKES NO WARRANTIES WHATSOEVER REGARDING THE ACCURACY OF THE CONFIDENTIAL INFORMATION, OTHER THAN AS SET FORTH EXPLICITLY IN THIS AGREEMENT, THE OTHER TRANSACTION DOCUMENTS, OR THE LICENSE AGREEMENT.

14.7 It is understood that no patent right or license is granted by this Agreement except for the Purpose and that the disclosure of Confidential Information does not result in any obligation by the Corporation to grant the Investor any right in and to such Confidential Information.

SECTION 15. Survival of Representations, Warranties and Agreements. The covenants, representations and warranties of the parties contained herein shall survive any SPA Closing hereunder. Each of the parties may rely on such covenants, representations and warranties irrespective of any investigation made, or notice or knowledge held by, it or any other person. All statements contained in any certificate or other instrument delivered by any party pursuant to this Agreement or in connection with the transactions contemplated by this Agreement shall constitute representations and warranties by such party under this Agreement, subject to the qualifications set forth herein and therein.

SECTION 16. Successors and Assigns. This Agreement shall be binding upon, and inure to the benefit of, each of the parties hereto and, except as otherwise expressly provided herein, each other person who shall become a registered holder named in a certificate evidencing Series C-1 Shares or Reserved Shares transferred to such holder by any of the Investor or its permitted transferees, and (except as aforesaid) their respective legal representatives, successors and assigns.

SECTION 17. Entire Agreement; Effect on Prior Documents. This Agreement and the other documents referred to herein or delivered pursuant hereto contain the entire agreement among the parties with respect to the financing transactions contemplated hereby and supersede all prior negotiations, commitments, agreements and understandings among them with respect thereto.

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SECTION 18. Notices. All notices, requests, consents and other communications hereunder (“Notices”) to any party shall be contained in a written instrument addressed to such party at the address set forth below or such other address as may hereafter be designated in writing by the addressee to the addressor listing all parties and shall be deemed given (a) when delivered in person or duly sent by fax showing confirmation of receipt, (b) three days after being duly sent by first class mail postage prepaid (other than in the case of Notices to or from any non-U.S. resident, which Notices must be sent in the manner specified in clause (a) or (c)), or (c) two days after being duly sent by DHL, Federal Express or other recognized express international courier service:

(a) if to the Corporation, to:

Acceleron Pharma Inc.

149 Sidney Street

Cambridge, MA 02139

with a copy to:

Ropes & Gray LLP

One International Place

Boston, MA 02110

Attn: Marc Rubenstein

Fax: (617) 951-7050

(b) if to the Investor, to its address as set forth on the signature page hereto.

(c) if to a Founders, to the address of such Founder set forth on the signature pages of this Agreement or to such Founder c/o the Corporation.

SECTION 19. Notice of Non-compliance. The Corporation shall promptly notify the Investor in writing if it has lost or has reason to believe it may lose any of its federal, state, local or foreign governmental licenses, registrations or permits necessary for or material to the conduct of its business, or if the Corporation receives any oral or written notice of any proceeding or inquiry that the Corporation reasonably believes may lead to the revocation or limitation of any of the foregoing licenses, registrations or permits.

SECTION 20. Amendments; Waivers. This Agreement may be amended, and compliance with the provisions of this Agreement may be omitted or waived, only by the written agreement of the Corporation and the Investor or assignees of its rights.

SECTION 21. Counterparts. This Agreement may be executed in any number of counterparts, each such counterpart shall be deemed to be an original instrument, and all such counterparts together shall constitute but one agreement. Any such counterpart may contain one or more signature pages.

SECTION 22. Headings. The headings of the various sections of this Agreement have been inserted for convenience of reference only and shall not be deemed to be a part of this Agreement.

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SECTION 23. Nouns and Pronouns. Whenever the context may require, any pronouns used herein shall include the corresponding masculine, feminine or neuter forms, and the singular form of names and pronouns shall include the plural and vice-versa.

SECTION 24. Governing Law. This Agreement shall be governed by, and construed and enforced in accordance with, the substantive laws of the Commonwealth of Massachusetts, without regard to its principles of conflicts of laws.

SECTION 25. Severability. Any provision of this Agreement that is prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction, be ineffective to the extent of such prohibition or unenforceability without invalidating the remaining provisions hereof, and any such prohibition or unenforceability in any jurisdiction shall not invalidate or render unenforceable such provision in any other jurisdiction.

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IN WITNESS WHEREOF, the undersigned have executed this Series C-1 Convertible Preferred Stock Purchase Agreement as of the day and year first written above.

ACCELERON PHARMA INC.
By:	/s/ John Knopf
	John Knopf, Chief Executive Officer

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series C-1 Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is the “Investor” as defined in the Series C-1 Convertible Preferred Stock Purchase Agreement dated as of February 20, 2008 among Acceleron Phanna Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 20th day of February 2008.

CELGENE CORPORATION (print name)
By:	/s/ Sol J. Barer
Name:	Sol J. Barer
Title:	Chairman and CEO
Address:
86 Morris Avenue
Summit, New Jersey, 07901

Signature Page to Stock Purchase Agreement

EX-4

Exhibit 4

SERIES E CONVERTIBLE PREFERRED STOCK

AND WARRANT PURCHASE AGREEMENT

THIS SERIES E CONVERTIBLE PREFERRED STOCK AND WARRANT PURCHASE AGREEMENT (“Agreement”) is made as of June 10, 2010 by and among Acceleron Pharma Inc., a Delaware corporation (the “Corporation”) and the investors that are named on the Initial Closing Schedule of Investors and Additional Closing Schedule of Investors attached hereto (the “Investors”).

WHEREAS, the Investors wish to purchase from the Corporation, and the Corporation wishes to sell to the Investors, up to an aggregate of 3,662,422 shares of the Corporation’s Series E Convertible Preferred Stock, $.001 par value per share (the “Series E Preferred Stock”) and common stock purchase warrants;

NOW, THEREFORE, in consideration of the premises and the mutual covenants and agreements herein contained, the parties hereby agree as follows:

SECTION 1. Amended and Restated Certificate of Incorporation. Prior to the Initial Closing (as defined in Section 3.1), the Corporation filed with the Secretary of State of the State of Delaware an Amended and Restated Certificate of Incorporation (the Certificate of Incorporation of the Corporation as so amended and restated and as in effect on the date hereof being hereinafter referred to as the “Certificate of Incorporation”), a copy of which is attached hereto as Exhibit 1.

SECTION 2. Purchase and Sale of the Series E Preferred Stock and Warrants.

2.1 Initial Series E Shares. Subject to the terms and conditions of this Agreement, at the Initial Closing, the Corporation agrees to issue and sell to the Investors identified on the Initial Closing Schedule of Investors the number of shares of Series E Preferred Stock (the “Initial Series E Shares”) set forth on the Initial Closing Schedule of Investors. Each Investor named on the Initial Closing Schedule of Investors, acting severally and not jointly, agrees to purchase from the Corporation at the Initial Closing the number of Initial Series E Shares, at the aggregate purchase price, set forth opposite the name of such Investor on the Initial Closing Schedule of Investors.

2.2 Initial Warrants. Subject to the terms and conditions of this Agreement, at the Initial Closing (as defined in Section 3.1), immediately following the issuance of the Initial Series E Shares, the Corporation agrees to issue to the Investors stock purchase warrants in the form attached hereto as Exhibit 2.2 (the “Warrants”) for the purchase of shares of Common Stock, $.001 par value per share (the “Warrant Shares”) in the number set forth on the Initial Schedule of Investors. Each Warrant may be exercised for Warrant Shares pursuant to the terms of the form attached hereto as Exhibit 2.2.

2.3 Additional Series E Shares and Warrants. After the Initial Closing, the Corporation shall undertake to provide notice to each stockholder of the Corporation listed on Exhibit 2.3 of its opportunity to purchase the number of shares of Series E Preferred Stock and

Warrants, at the aggregate purchase price, set forth opposite the name of such stockholder on Exhibit 2.3. Any shares of Series E Preferred Stock and Warrants set forth on Exhibit 2.3 that remain unsubscribed at the expiration of the time frame set forth by the Corporation in such notice (which time frame shall be determined by the Corporation in its discretion) shall be offered for purchase to all other Investors (including Additional Closing Investors), other than Undersubscribing Stockholders (as defined below), on a time frame and on conditions to be set forth by the Corporation in its discretion. “Undersubscribing Stockholder” means any stockholder that does not duly elect to purchase the full number of shares of Series E Preferred Stock and Warrants, if any, set forth opposite the name of such stockholder on Exhibit 2.3. Any stockholder duly electing to purchase all or a portion of its Series E Preferred Stock and Warrants set forth on Exhibit 2.3 and any Investor identified on the Initial Closing Schedule of Investors that duly elects to purchase any unsubscribed shares of Series E Preferred Stock and Warrants shall be deemed an “Additional Closing Investor,” and, collectively, the “Additional Closing Investors.” All Additional Closing Investors and the number of shares of Additional Series E Shares (as defined in Section 3.2) and Warrants to be sold to each Additional Closing Investor at the Additional Closing (as defined in Section 3.2) shall be reflected on the Additional Closing Schedule of Investors, which shall be automatically amended by the Corporation without any further action by any other party hereto to reflect the elections described above. Each Additional Closing Investor, acting severally and not jointly, agrees to purchase from the Corporation at the Additional Closing the number of shares of Series E Preferred Stock, at the aggregate purchase price, set forth opposite the name of such Additional Closing Investor on the Additional Closing Schedule of Investors.

2.4 Additional Warrants. Subject to the terms and conditions of this Agreement, at the Additional Closing, immediately following the issuance of the Additional Series E Shares, the Corporation agrees to issue to the Investors Warrants for the purchase of Warrant Shares in the number set forth on the Additional Closing Schedule of Investors. Each Warrant may be exercised for Warrant Shares pursuant to the terms of the form attached hereto as Exhibit 2.2.

SECTION 3. Closing.

3.1 Initial Closing. The closing of the sale and purchase of the Initial Series E Shares and Warrants (the “Initial Closing”) shall take place upon the execution of this Agreement at the offices of Ropes & Gray LLP, One International Place, Boston, Massachusetts, or at such other location as may be agreed upon among the Investors and the Corporation. At the Initial Closing, the Corporation shall issue and deliver to each Investor a certificate or certificates for shares of Series E Preferred Stock, registered in the name of such Investor, in the amount representing the number of Initial Series E Shares being purchased by such Investor and a stock purchase warrant representing the Warrants being purchased by such Investor at the Initial Closing, against payment by such Investor to the Corporation of the aggregate purchase price therefor in the form of (a) a certified or bank check payable to the order of the Corporation, (b) a wire transfer to a bank account designated by the Corporation or (c) a combination of (a) and (b).

3.2 Additional Closing. After the Initial Closing, the Corporation may sell, on the same terms and conditions as those contained in this Agreement, up to 583,337 additional shares of Series E Preferred Stock (the “Additional Series E Shares”, and together with the Initial Series E Shares and any shares of Series E Preferred Stock sold pursuant to the last sentence of this

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Section 3.2, collectively the “Series E Shares”) and Warrants, to the electing Investors pursuant to Section 2.2 (the “Additional Closing” and, together with the Initial Closing, each a “Closing”), provided that (i) such subsequent sale is consummated on or prior to sixty (60) days after the Initial Closing and (ii) each Additional Closing Investor that did not purchase Series E Shares at the Initial Closing shall become a party to the Transaction Documents (as defined below), by executing and delivering a counterpart signature page to each of the Transaction Documents. At the Additional Closing, the Corporation shall issue and deliver to each applicable Investor a certificate or certificates for shares of Series E Preferred Stock, registered in the name of such Additional Closing Investor, in the amount representing the Additional Series E Shares being purchased by such Additional Closing Investor at the Additional Closing, against payment by such Additional Closing Investor to the Corporation of the aggregate purchase price therefor in the form of (a) a certified or bank check payable to the order of the Corporation, (b) a wire transfer to a bank account designated by the Corporation or (c) a combination of (a) and (b). In addition to the foregoing, at the Additional Closing the Corporation may also sell up to 477,707 additional shares of Series E Preferred Stock and Warrants to purchase 510,204 Warrant Shares to Oxford Finance Corporation, SVB Financial Group and MidCap Funding III, LLC (or their respective Affiliates), collectively, pursuant to that certain Amended and Restated Right to Invest Letter Agreement, dated March 18, 2010 (the “RTI Letter”), and any such purchasers shall be deemed Additional Closing Investors hereunder and subject to all obligations of Additional Closing Investors. For clarity, if such shares of Series E Preferred Stock and Warrants are not elected to be purchased by Oxford Finance Corporation, SVB Financial Group and MidCap Funding III, LLC (or their respective Affiliates), such shares and warrants will not be reallocated to the Investors pursuant to Section 2.3 or otherwise sold other than in compliance with the Certificate of Incorporation and Transaction Documents.

SECTION 4. Representations and Warranties of the Corporation. The Corporation represents and warrants to the Investors as follows:

4.1 Organization. The Corporation is a corporation duly organized, validly existing and in good standing under the laws of the State of Delaware and has all requisite corporate power and authority to own and lease its properties, to carry on its business as presently conducted and as proposed to be conducted by it and to carry out the transactions contemplated by this Agreement. The Corporation is duly qualified as a foreign corporation and is in good standing in all such jurisdictions (which jurisdictions are listed in Schedule 4.1A) in which the conduct of its business or its ownership or leasing of property requires such qualification, except to the extent that any failure to be so qualified would not materially and adversely affect the financial condition, results of operations, assets, liabilities business or prospects of the Corporation.

4.2 Capitalization. Immediately prior to the Initial Closing, the entire authorized capital stock of the Corporation consists of:

(a) 91,515,101 shares of Common Stock, $.001 par value per share (“Common Stock”), of which (i) 8,936,563 shares have been issued and are outstanding, fully paid and nonassessable; (ii) no shares are held as treasury shares; (iii) 13,400,000 have been

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reserved for issuance upon exercise of options granted or to be granted under stock purchase, stock option or other equity incentive plans of the Corporation; (iv) 26,069,980 shares have been reserved for issuance upon conversion of the outstanding shares of Series A Preferred Stock of the Corporation and shares of Series A Preferred Stock issuable upon exercise of outstanding warrants; (v) 16,944,378 shares have been reserved for issuance upon conversion of the Series B Preferred Stock and shares of Series B Preferred Stock issuable upon exercise of outstanding warrants; (vi) 11,923,077 shares have been reserved for issuance upon conversion of the Series C Preferred Stock; (vii) 2,014,652 shares have been reserved for issuance upon conversion of the Series C-1 Preferred Stock and shares of Series C-1 Preferred Stock issuable upon exercise of outstanding warrants; (viii) 955,414 shares have been reserved for issuance upon conversion of the Series D Preferred Stock; (ix) 2,802,548 shares have been reserved for issuance upon conversion of the Series D-1 Preferred Stock; and (x) 7,573,989 shares have been reserved for issuance upon conversion of the Series E Preferred Stock and upon exercise of outstanding Warrants;

(b) 26,069,980 of Series A Preferred Stock, 25,643,980 shares of which are issued and outstanding;

(c) 16,944,378 shares of Series B Preferred Stock, 16,816,810 of which are issued and outstanding;

(d) 11,923,077 shares of Series C Preferred Stock, 11,912,333 shares of which are issued and outstanding;

(e) 2,014,652 shares of Series C-1 Preferred Stock, 1,831,502 shares of which are issued and outstanding;

(f) 955,414 shares of Series D Preferred Stock, 939,847 of which are issued and outstanding;

(g) 2,802,548 shares of Series D-1 Preferred Stock, 2,547,771 of which are issued and outstanding; and

(h) 3,662,422 shares of Series E Preferred Stock, 2,660,962 of which are being issued at the Initial Closing and immediately thereafter will be issued and outstanding, fully paid and nonassessable and will be held by the Investors and the remaining of which are reserved for issuance at the Additional Closing or otherwise in accordance with the terms of this Agreement.

Schedule 4.2 contains a list of all holders of capital stock of the Corporation and options, warrants or rights to purchase such capital stock that will be outstanding immediately before the Initial Closing, in each case including the number of shares of capital stock held by, or subject to purchase pursuant to the exercise of any option, warrant or right held by, each such holder. Except as set forth in Schedule 4.2, there are no outstanding shares of capital stock of the Corporation or warrants, options, agreements, convertible securities or other commitments pursuant to which the Corporation is or may become obligated to issue any shares of its capital stock or other securities as of the Initial Closing. Except as set forth in Schedule 4.2, the number of shares of capital stock, if any, issuable in connection with the securities described in the first

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sentence of this paragraph is not subject to adjustment by reason of the issuance of the Series E Shares, the Warrants or the Reserved Shares (as defined in Section 4.22). There are no preemptive or similar rights to purchase or otherwise acquire shares of capital stock of the Corporation from the Corporation pursuant to any provision of law, the Certificate of Incorporation or the By-Laws or, except as set forth in Schedule 4.2, any agreement to which the Corporation is a party, or otherwise. Except as set forth in the Amended and Restated Registration Rights Agreement, Amended and Restated Voting Agreement, Amended and Restated Right of First Refusal and Co-Sale Agreement and Amended and Restated Investor Rights Agreement (each as defined in Section 4.22) or in Schedule 4.2, there is no agreement, restriction or encumbrance with respect to the sale or voting of any shares of the Corporation’s capital stock (whether outstanding or issuable upon conversion or exercise of outstanding securities). The Corporation has not violated the Securities Act of 1933, as amended (the “Securities Act”) or any securities law of any state or other jurisdiction in connection with the issuance of any securities.

4.3 Equity Investments. Except as set forth in Schedule 4.3, the Corporation does not currently own, directly or indirectly, any capital stock or other proprietary interest in any corporation, association, trust, partnership, limited liability company, limited liability partnership, joint venture or other entity.

4.4 Financial Statements. Attached as Schedule 4.4 is the unaudited consolidated balance sheet of the Corporation (the “Balance Sheet”) as of March 31, 2010 (the “Balance Sheet Date”). The Balance Sheet (a) is true and correct in all material respects, (b) is in accordance with the books and records of the Corporation, and (c) presents fairly the financial position of the Corporation as of the Balance Sheet Date.

4.5 Absence of Undisclosed Liabilities. Except as set forth in Schedule 4.5 or as reflected on the Balance Sheet, the Corporation has no liabilities of any nature (matured or unmatured, fixed or contingent) that individually or in the aggregate exceed $50,000.

4.6 Absence of Changes. Except as set forth in Schedule 4.6, since the Balance Sheet Date there has not been (a) any change in the financial condition, results of operations, assets, liabilities, business or prospects of the Corporation that, individually or in the aggregate, is materially adverse, (b) any material asset or property of the Corporation made subject to a lien of any kind, except liens for taxes not yet due and payable, (c) any satisfaction or discharge of any lien, claim, or encumbrance or payment of any obligation by the Corporation, except in the ordinary course of business and that is not material to the financial condition, results of operations, assets, liabilities, business or prospects of the Corporation, (d) any waiver of any valuable right of the Corporation, or the cancellation of any debt or claim held by the Corporation, (e) any payment of dividends on, or other distribution with respect to, or any direct or indirect redemption or acquisition of, any shares of the capital

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stock of the Corporation, or any agreement or commitment therefor, (f) any mortgage, pledge, sale, assignment or transfer of any tangible or intangible assets of the Corporation, except in the ordinary course of business, (g) any loan by the Corporation to, or any loan to the Corporation from, any officer, director, employee or stockholder of the Corporation, or any agreement or commitment therefor, (h) any damage, destruction or loss (whether or not covered by insurance) materially and adversely affecting the assets, property or business of the Corporation, (i) any change in the accounting methods or practices followed by the Corporation, (j) any amendment to the Corporation’s Certificate of Incorporation or Bylaws, except as provided in this Agreement or (k) any agreement or commitment by the Corporation to do any of the things described in this Section 4.6. Each of the occurrences described in clauses (a) through (k) above is sometimes hereinafter referred to as a “Material Adverse Change.”

4.7 Encumbrances. The Corporation has good and marketable title to all of its property and assets, real, personal or mixed, tangible or intangible, free and clear of all liens, security interests, charges and other encumbrances of any kind, except (i) liens for taxes not yet due and payable, (ii) liens securing debt reflected on the Balance Sheet, and (iii) liens arising in the ordinary course of business.

4.8 Permits. The Corporation has all franchises, permits, licenses, authorizations, approvals, certificates, consents, orders and any similar authority (collectively, the “Permits”) from state, federal and regulatory authorities of the United States, including without limitation, the United States Food and Drug Administration (the “FDA”), necessary for the conduct of its business as now being conducted by it, except for those the lack of which would not reasonably be expected to cause a Material Adverse Change. All such Permits are valid and in full force and effect. There is no FDA enforcement action pending or, to the knowledge of the Corporation, threatened against the Corporation. The Corporation is not in default in any material respect under any of such franchises, permits, licenses or other similar authority.

4.9 Intellectual Property Rights.

(a) All of the patents, trademarks, service marks, trade names, copyrights, applications for patents, trademarks, service marks and copyrights owned or licensed exclusively by the Corporation are listed in Schedule 4.9 (the “Scheduled Intellectual Property”);

(i) Except as set forth in Schedule 4.9, no royalties or other amounts are payable by the Corporation to other persons by reason of the ownership or use of the Scheduled Intellectual Property or the other Intellectual Property Rights (as defined below);

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(ii) To the knowledge of the Corporation and except as set forth on Schedule 4.9, (i) no product or service marketed or sold by the Corporation and no Intellectual Property Rights licensed or proposed to be licensed by the Corporation as licensor violates any license or infringes any Intellectual Property Rights of another, and (ii) the Corporation has not received any notice that any of the Intellectual Property Rights identified in Schedule 4.9 or the operation of the business of the Corporation conflicts or will conflict with the Intellectual Property Rights of others; and

(b) There are no claims pending or threatened with respect to any Intellectual Property Rights necessary or required for the conduct of the business of the Corporation as currently conducted, nor, to the Corporation’s knowledge, does there exist any basis therefor.

As used herein, the term “Intellectual Property Rights” means all patents, trademarks, service marks, trade names, copyrights, inventions, trade secrets, know-how, proprietary processes and formulae, applications for patents, trademarks, service marks and copyrights, and other industrial and intellectual property rights.

4.10 Litigation. Except as set forth in Schedule 4.10, there is no action, suit, claim, proceeding or investigation, at law, in equity or otherwise, or by or before any governmental instrumentality or other agency, now pending or, to the knowledge of the Corporation, threatened against or affecting the Corporation, nor, to the knowledge of the Corporation, does there exist any basis therefor.

4.11 No Defaults. Except as set forth in Schedule 4.11, the Corporation is not in violation or breach of, or in default under, any provision of the Certificate of Incorporation or By-Laws, and there exists no condition, event or act which after notice, lapse of time, or both, may constitute a violation or breach of, or a default under, the Certificate of Incorporation or By-Laws. The Corporation is not in violation or breach of, or in default under any note, indenture, mortgage, lease, contract, purchase order or other instrument, document or agreement to which the Corporation is a party or by which it or any of its property is bound or affected or any ruling, writ, injunction, order, judgment or decree of any court, administrative agency or other governmental body, except to the extent that any of the foregoing individually or in the aggregate would not result in a Material Adverse Change. Except as set forth in Schedule 4.11, to the knowledge of the Corporation, there exists no condition, event or act which after notice, lapse of time, or both, may constitute a violation or breach of, or a default under, any of the foregoing.

4.12 Employment of Founders, Officers, Employees and Consultants. To the knowledge of the Corporation and except as set forth on Schedule 4.12, no employee of the Corporation is in violation of any term of any contract or covenant (either with the Corporation or with another entity) relating to employment, consulting, assignment of inventions, proprietary information disclosure, non-competition or non-solicitation. To the

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knowledge of the Corporation and except as set forth on Schedule 4.12, neither of Jasbir Seehra or John Knopf (the “Founders”) is obligated under any contract or subject to any judgment, decree or administrative order that would conflict or interfere with (i) the performance of such Founder’s duties as an employee, consultant, director or officer of the Corporation, as the case may be, or (ii) the Corporation’s business as conducted or proposed to be conducted.

4.13 Taxes. The Corporation has filed all federal, state, local and foreign tax returns which are required to be filed by it and all such returns are true and correct in all material respects. The Corporation has paid all taxes shown as due thereon, or pursuant to any assessments received by it or which it is obligated to withhold from amounts owing to any employee, creditor or third party, except, in each case, for those taxes which are not yet due and payable pursuant to such returns.

4.14 Material Agreements. Except as set forth in Schedule 4.14, the Corporation is not a party to any written or oral (a) contract with any labor union; (b) contract for the future purchase of fixed assets or materials, supplies or equipment in excess of normal operating requirements; (c) contract for the employment of any officer, employee or other person or any contract with any person on a consulting basis; (d) bonus, pension, profit-sharing, retirement, stock purchase, stock option, hospitalization, medical insurance or similar plan, contract or understanding in effect with respect to employees or any of them or the employees of others; (e) agreement or indenture relating to the borrowing of money or to the mortgaging, pledging or otherwise placing a lien on any assets of the Corporation; (f) guaranty of any obligation for borrowed money or otherwise; (g) lease or agreement under which the Corporation is lessee of or holds or operates any property, real or personal, owned by any other party; (h) lease or agreement under which the Corporation is lessor of or permits any third party to hold or operate any property, real or personal, owned or controlled by the Corporation; (i) license or lease agreement with respect to any Intellectual Property Rights; (j) agreement or other commitment for capital expenditures in excess of $50,000; (k) contract, agreement or commitment under which the Corporation is obligated to pay any broker’s fees, finder’s fees or any such similar fees, to any third party; or (l) any other contract, agreement, arrangement or understanding which is material to the business of the Corporation. Each contract, agreement, arrangement, understanding or commitment listed on Schedule 4.14 (each a “Material Agreement”) is a valid and binding obligation of the Corporation, and, to the knowledge of the Corporation, each other party thereto, enforceable against the Corporation in accordance with its terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a

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proceeding in equity or at law)), and has been duly executed and delivered by the Corporation and, to the knowledge of the Corporation, each other party thereto. With respect to each Material Agreement, (a) the Corporation and, to the knowledge of the Corporation, each other party to such agreement has performed in all material respects all obligations required to be performed to date under such Material Agreement; (b) to the knowledge of the Corporation, no party to such Material Agreement is in default or arrears under the terms of such Material Agreement; and (c) to the knowledge of the Corporation, no condition exists or event has occurred that, with the giving of notice or lapse of time or both, would constitute a default under such Material Agreement. The Corporation has furnished to the Investors true and correct copies of all such written agreements and other documents, in each case to the extent requested by the Investors or their authorized representatives.

4.15 ERISA. Except as set forth on Schedule 4.15, neither the Corporation nor any entity required to be aggregated with the Corporation under Sections 414(b), (c), (m) or (n) of the Internal Revenue Code of 1986, as amended (the “Code”), sponsors, maintains, has any obligation to contribute to, has any liability under, or is otherwise a party to, any Benefit Plan. For purposes of this Agreement, “Benefit Plan” shall mean any plan, fund, program, policy, arrangement or contract, whether formal or informal, which is in the nature of (i) an employee pension benefit plan (as defined in Section 3(2) of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”)) or (ii) an employee welfare benefit plan (as defined in section 3(1) of ERISA).

4.16 U.S. Real Property Holding Corporation. The Corporation is not now and has never been a “United States real property holding corporation,” as defined in Section 897(c)(2) of the Code and Section 1.897-2(b) of the Treasury Regulations promulgated by the Internal Revenue Service, and the Corporation has never filed with the Internal Revenue Service a statement with its United States income tax returns under Section 1.897-2(h) of such Regulations stating that any shares of its capital stock constitute a U.S. real property interest within the meaning of Section 897(c)(1) of the Code.

4.17 Environmental Protection. Except as set forth on Schedule 4.17, the Corporation has not caused or allowed, or contracted with any party for, the generation, use, transportation, treatment, storage or disposal of any Hazardous Substances (as defined below) in connection with the operation of its business or otherwise. The Corporation does not presently own any real property. The operation of the business of the Corporation is in compliance with all applicable Environmental Laws (as defined below) and orders or directives of any governmental authorities having jurisdiction under such Environmental Laws, including, without limitation, any Environmental Laws or orders or directives with respect to any

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cleanup or remediation of any release or threat of release of Hazardous Substances. The Corporation has not received any citation, directive, letter or other communication, written or oral, or any notice of any proceeding, claim or lawsuit, from any person arising out of the conduct of its operations, and the Corporation is not aware of any basis therefor. The Corporation has obtained and is maintaining in full force and effect all necessary permits, licenses and approvals required by all Environmental Laws applicable to the business operations conducted by the Corporation, and is in compliance with all such permits, licenses and approvals. For the purposes of this Agreement, the term “Environmental Laws” shall mean any Federal, state or local law or ordinance or regulation pertaining to the protection of human health or the environment, including, without limitation, the Comprehensive Environmental Response, Compensation, and Liability Act, 42 U.S.C. Sections 9601, et seq., the Emergency Planning and Community Right-to-Know Act, 42 U.S.C. Sections 11001, et seq., and the Resource Conservation and Recovery Act, 42 U.S.C. Sections 6901 et seq. For purposes of this Agreement, the term “Hazardous Substances” shall include oil and petroleum products, asbestos, polychlorinated biphenyls, urea formaldehyde and other materials classified as hazardous or toxic under any Environmental Laws.

4.18 Foreign Corrupt Practices Act. The Corporation has not taken any action which would cause it to be in violation of the Foreign Corrupt Practices Act of 1977, as amended, or any rules and regulations thereunder. There is not now, and there has never been, any employment by the Corporation of, or beneficial ownership in the Corporation by, any governmental or political official in any country in the world.

4.19 Federal Reserve Regulations. The Corporation is not engaged in the business of extending credit for the purpose of purchasing or carrying margin securities (within the meaning of Regulation U of the Board of Governors of the Federal Reserve System), and no part of the proceeds of the sale of Series E Shares or Warrants will be used to purchase or carry any margin security or to extend credit to others for the purpose of purchasing or carrying any margin security or in any other manner which would involve a violation of any of the regulations of the Board of Governors of the Federal Reserve System.

4.20 Compliance. The Corporation has complied in all material respects with all federal, state, local and foreign laws applicable to its business. The Corporation has all federal, state, local and foreign governmental licenses, registrations and permits necessary for the conduct of its business, except for those the absence of which would not materially and adversely affect the financial condition, results of operations, assets, liabilities business or prospects of the Corporation; such licenses, registrations and permits are in full force and effect; and there have been no violations of any such licenses, registrations or permits except for violations that individually or

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in the aggregate would not result in a Material Adverse Change. Except as set forth on Schedule 4.20, no proceeding is pending, nor has the Corporation received any oral or written notice of any proceeding that is threatened, to revoke or limit any of the foregoing licenses, registrations or permits. Without limiting the generality of the foregoing, the Corporation has complied in all material respects with all applicable laws relating to the employment of labor, including provisions relating to wages, hours, equal opportunity, collective bargaining and the payment of Social Security and other taxes, and with ERISA.

4.21 Insurance. Schedule 4.21 sets forth each insurance policy (specifying the insurer, the amount of coverage, the type of insurance, the policy number, the expiration date, the annual premium and any pending claims thereunder) maintained by the Corporation on its properties, assets, business or personnel. No notice from any insurance carrier has been received by the Corporation claiming that the Corporation is in default with respect to any provision contained in any insurance policy listed on Schedule 4.21.

4.22 Authorization of Transaction Documents. The execution, delivery and performance by the Corporation of (a) this Agreement, (b) the Amended and Restated Registration Rights Agreement of even date herewith by and among the Corporation and the Investors and certain other parties in the form of Exhibit 4.22A (the “Amended and Restated Registration Rights Agreement”), (c) the Amended and Restated Voting Agreement of even date herewith by and among the Corporation and the other parties thereto in the form of Exhibit 4.22B (the “Amended and Restated Voting Agreement”), (d) the Amended and Restated Right of First Refusal and Co-Sale Agreement of even date herewith by and among the Corporation and the other parties thereto in the form of Exhibit 4.22C (the “Amended and Restated Right of First Refusal and Co-Sale Agreement”), and (e) the Amended and Restated Investor Rights Agreement of even date herewith by and among the Corporation and the other parties thereto in the form of Exhibit 4.22D (the “Amended and Restated Investor Rights Agreement” and, together with this Agreement, the Amended and Restated Registration Rights Agreement, the Amended and Restated Voting Agreement, the Amended and Restated Right of First Refusal and Co-Sale Agreement and the Amended and Restated Investor Rights Agreement, the “Transaction Documents”) have been duly authorized by all requisite corporate action. The Corporation has duly authorized, executed and delivered each Transaction Document, and each Transaction Document constitutes the valid and binding obligation of the Corporation, enforceable in accordance with its terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)). The

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execution, delivery and performance of the Transaction Documents, the issuance, sale and delivery of the Series E Shares, the Warrants and the shares of Common Stock issuable upon conversion of the Series E Shares or exercise of the Warrants (the “Reserved Shares”), and compliance with the provisions hereof and thereof by the Corporation do not and will not, with or without the passage of time or the giving of notice or both, violate, conflict with or result in any breach of any of the terms, conditions or provisions of, or constitute a default (or give rise to any right of termination, cancellation or acceleration) under, or result in the creation of any lien, security interest, charge or encumbrance upon any of the properties or assets of the Corporation under, the Certificate of Incorporation or Bylaws, any Material Agreement, or any provision of law, statute, rule or regulation or any ruling, writ, injunction, order, judgment or decree of any court naming the Corporation, or administrative agency or other governmental body.

4.23 Authorization of Series E Shares, Warrants and Reserved Shares. The issuance, sale and delivery hereunder by the Corporation of the Series E Shares and Warrants have been duly authorized by all requisite corporate action of the Corporation, and when so issued, sold and delivered the Series E Shares will be validly issued and outstanding, fully paid and nonassessable, and not subject to preemptive or any other similar rights of the stockholders of the Corporation or others. The issuance and delivery of the Reserved Shares have been duly authorized by all requisite corporate action of the Corporation, and the Reserved Shares have been duly reserved for issuance upon conversion of any or all of the Series E Shares or exercise of any or all of the Warrants, and when so issued and delivered upon conversion of the Series E Shares or exercise of the Warrants, the Reserved Shares will be validly issued and outstanding, fully paid and nonassessable, and not subject to preemptive or any other similar rights of the stockholders of the Corporation or others.

4.24 Related Transactions. Except as set forth in Schedule 4.24, no director, officer or employee of the Corporation nor any “associate” (as defined in Rule 12b-2 promulgated under the Securities Exchange Act of 1934, as amended (the “Exchange Act”)) of any such person is indebted to the Corporation, nor is the Corporation indebted (or committed to make loans or extend or guarantee credit) to any such person, nor is any such person a party to any transaction (other than as an employee or consultant) with the Corporation providing for the furnishing of services by, or rental of real or personal property from, or otherwise requiring cash payments to, any such person.

4.25 Offerees. Except as set forth in Schedule 4.25, during the six months prior to the date of this Agreement, the Corporation has not, either directly or through any agent, offered any Common Stock, Warrants, Series E Preferred Stock or other securities convertible into Common Stock,

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Warrants, Series E Preferred Stock or any security or securities similar to any thereof, for sale to, or solicited any offers to buy any Common Stock, Warrants, Series E Preferred Stock or other securities convertible into Common Stock, Warrants, Series E Preferred Stock or any such similar security or securities from, or otherwise approached or negotiated in respect thereof with, any person or entity other than the Investors and a limited number of institutional or sophisticated investors.

4.26 No Governmental Consent or Approval Required. No authorization, consent, approval or other order of, declaration to, or filing with, any governmental agency or body is required to be made or obtained by the Corporation for or in connection with the valid and lawful authorization, execution and delivery by the Corporation of the Transaction Documents, for or in connection with the valid and lawful authorization, issuance, sale and delivery of the Series E Shares and Warrants or for or in connection with the valid and lawful authorization, reservation, issuance, sale and delivery of the Reserved Shares, except the exemptive filings under applicable securities laws set forth on Schedule 4.26, which are not required to be made until after the Closing and which shall be made on a timely basis.

4.27 Registration Rights. Except as contemplated by the Amended and Restated Registration Rights Agreement, no person has any right to cause the Corporation to effect the registration under the Securities Act of any shares of Common Stock or any other securities of the Corporation.

4.28 Employees. Except as set forth on Schedule 4.28, each employee or consultant now employed or engaged by the Corporation who has access to confidential or proprietary information of the Corporation has executed an agreement with the Corporation relating to, among other matters, non-disclosure of confidential and proprietary information and the assignment of intellectual property, each of which agreements is in full force and effect. The Corporation has complied in all material respects with all applicable state and federal equal opportunity, minimum wage, immigration, workforce reduction and other laws related to employment and termination of employment, except in those cases where such non-compliance would not reasonably be expected to cause a Material Adverse Change. No officer or key employee of the Corporation has advised the Corporation (orally or in writing) that he intends to terminate employment with the Corporation.

4.29 Exemptions from Securities Laws. Subject to the accuracy of the representations and warranties of the Investors set forth in Section 5 hereof, the provisions of Section 5 of the Securities Act are inapplicable to the offering, issuance, sale and delivery of the Series E Shares, Warrants and the Reserved Shares, and no consent, approval, qualification or registration or filing under any state securities laws is required in connection therewith, except the exemptive filings set forth on Schedule 4.26, which are not required to be made until after the Closing and which shall be made on a timely basis.

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4.30 Investment Company Act. The Corporation is not an “investment company,” or a company “controlled” by an “investment company,” within the meaning of the Investment Company Act of 1940, as amended.

4.31 83(b) Elections. To the best of the Corporation’s knowledge, all individuals who have purchased unvested shares of the Corporation’s Common Stock have timely filed elections under Section 83(b) of the Code.

4.32 Disclosure. Neither this Agreement nor any other certificate, document or written statement furnished to the Investors by or on behalf of the Corporation contains any untrue statement of a material fact or omits to state a material fact necessary in order to make the statements contained herein or therein not misleading.

SECTION 5. Representations and Warranties of the Investors. Each of the Investors, severally and not jointly, represents and warrants to the Corporation as follows:

5.1 Purchase for Investment. Such Investor is acquiring the Series E Shares and Warrants purchasable by it hereunder for its own account, for investment and not for, with a view to, or in connection with, any distribution or public offering thereof within the meaning of the Securities Act.

5.2 Unregistered Securities; Legend. Such Investor understands that the Series E Shares, the Warrants and the Reserved Shares have not been, and will not be, registered under the Securities Act or any state securities law, by reason of their issuance in a transaction exempt from the registration requirements of the Securities Act and such laws, that the Series E Shares, the Warrants and the Reserved Shares must be held indefinitely unless they are subsequently registered under the Securities Act and such laws or a subsequent disposition thereof is exempt from registration, that the certificates for the Series E Shares and the Reserved Shares shall bear a legend to such effect, and that appropriate stop transfer instructions may be issued. Such Investor further understands that such exemption depends upon, among other things, the bona fide nature of such Investor’s investment intent expressed herein.

5.3 Status of the Investors. Such Investor has not been formed for the specific purpose of acquiring the Series E Shares or the Warrants pursuant to this Agreement. Such Investor understands the term “accredited investor” as used in Regulation D promulgated under the Securities Act and represents and warrants to the Corporation that such Investor is an “accredited investor” for purposes of acquiring the Series E Shares and Warrants purchasable by it hereunder.

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5.4 Knowledge and Experience; Economic Risk. Such Investor has sufficient knowledge and experience in business and financial matters and with respect to investment in securities of privately held companies so as to enable it to analyze and evaluate the merits and risks of the investment contemplated hereby and is capable of protecting its interest in connection with this transaction. Such Investor is able to bear the economic risk of such investment, including a complete loss of the investment.

5.5 Access to Information. Such Investor acknowledges that such Investor and its representatives have had the opportunity to ask questions and receive answers from officers and representatives of the Corporation concerning the Corporation and its business and the transactions contemplated by this Agreement and to obtain any additional information which the Corporation possesses or can acquire that is necessary to verify the accuracy of the information regarding the Corporation herein set forth or otherwise desired in connection with its purchase of the Series E Shares and Warrants purchasable by it hereunder.

5.6 Rule 144. Such Investor understands that the exemption from registration afforded by Rule 144 (the provisions of which are known to such Investor) promulgated by the Securities and Exchange Commission under the Securities Act depends upon the satisfaction of various conditions, and that such exemption is not currently available.

5.7 Place of Business. Such Investor has listed its principal place of business or registered address next to its name on the signature page hereto.

5.8 Authorization of Transaction Documents. Such Investor has duly authorized, executed and delivered the Transaction Documents to which such Investor is a party, and such Transaction Documents constitute the valid and binding obligation of such Investor, enforceable against such Investor in accordance with their terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)).

SECTION 6. Conditions Precedent to Closing by the Investors.

6.1 Initial Closing. The obligation of each Investor to purchase and pay for the Series E Shares and Warrants being purchased by such Investor at the Initial Closing is subject to satisfaction (or waiver by such Investor) of the following conditions precedent at or before the Initial Closing:

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(a) Corporate Proceedings. All corporate and other proceedings to be taken and all waivers and consents to be obtained in connection with the transactions contemplated by this Agreement shall have been taken or obtained and all documents incident to such transactions shall be reasonably satisfactory in form and substance to the Investors and their counsel, who shall have received all such originals or certified or other copies of such documents as they may reasonably request.

(b) Representations and Warranties Correct. The representations and warranties made by the Corporation in Section 4 hereof (each as modified by any schedule referred to therein) shall be true and correct when made, and shall be true and correct in all material respects at the time of the Initial Closing with the same force and effect as if they had been made at and as of the time of the Initial Closing.

(c) Compliance with Covenants. The Corporation shall have duly complied with and performed all covenants and agreements of the Corporation herein which are required to be complied with and performed at or before the Initial Closing.

(d) Certificate of Compliance. The Corporation shall have provided to the Investors a certificate, dated the date of the Initial Closing, in form and substance reasonably satisfactory to the Investors, confirming compliance with the conditions set forth in Sections 6.1(b) and 6.1(c).

(e) Secretary’s Certificate. The Secretary or an Assistant Secretary of the Corporation shall have delivered to the Investors a certificate, dated the date of the Initial Closing, certifying: (a) that attached thereto is a true and complete copy of the By-Laws of the Corporation as in effect on the date of such certification; (b) that attached thereto is a true and complete copy of all resolutions adopted by the Board of Directors and the stockholders of the Corporation authorizing the transactions contemplated by this Agreement and any other actions necessary thereto; (c) that attached thereto is a true and complete copy of the Certificate of Incorporation as in effect on the date of such certification; and (d) to the incumbency and signatures of the officers of the Corporation executing the Transaction Documents.

(f) Opinion of Counsel. Each of the Investors purchasing Initial Series E Shares shall have received an opinion of Ropes & Gray LLP, counsel for the Corporation, addressed to such Investors, in substantially the form attached hereto as Exhibit 6.1(f).

(g) Related Agreements and Documents. At or before the Initial Closing, the parties thereto shall have executed and delivered the Transaction Documents, and the Corporation shall have delivered to the Investors such other documents consistent with the terms hereof as they shall reasonably request.

(h) Securities Matters. All consents, approvals, qualifications, registrations, notices and filings required to be obtained or effected as of the Initial Closing under any applicable securities laws of any state or other jurisdiction in connection with the issuance, sale and delivery of the Series E Shares, the Warrants and the Reserved Shares shall have been obtained or effected and copies of the same delivered to each of the Investors.

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(i) Management Rights Letters. The Corporation shall have executed and delivered to each Investor that so requests a management letter substantially in the form attached hereto as Exhibit 6.1(i).

(j) Delivery of Certificates for Series E Shares. The Corporation shall have delivered to each Investor a certificate for the number and type of Series E Shares being purchased by such Investor at the Initial Closing, each registered in the name of such Investor.

(k) Delivery of Warrants. The Corporation shall have executed and delivered to each Investor a Warrant substantially in the form attached hereto as Exhibit 2.2 at the Initial Closing, issued in the name of such Investor in accordance with Section 2.2.

6.2 Additional Closing. The obligation of each Additional Closing Investor to purchase and pay for the Series E Shares and Warrants being purchased by such Additional Closing Investor at the Additional Closing is subject to satisfaction (or waiver by such Additional Closing Investor) of the following conditions precedent at or before the Additional Closing:

(a) Corporate Proceedings. All corporate and other proceedings to be taken and all waivers and consents to be obtained in connection with the transactions contemplated by this Agreement shall have been taken or obtained and all documents incident to such transactions shall be reasonably satisfactory in form and substance to the Additional Closing Investors and their counsel, who shall have received all such originals or certified or other copies of such documents as they may reasonably request.

(b) Representations and Warranties Correct. The representations and warranties made by the Corporation in Section 4 (each as modified by any schedule referred to therein) shall be true and correct when made, and shall be true and correct in all material respects at the time of the Additional Closing with the same force and effect as if they had been made at and as of the time of the Additional Closing, except as may be set forth in a supplemental disclosure schedule (the “Supplemental Disclosure Schedule”) delivered by the Corporation to the Additional Closing Investors in connection with the Additional Closing; provided, that the delivery of a Supplemental Disclosure Schedule shall in no way relieve an Investor of its obligation to purchase Series E Shares at the Additional Closing.

(c) Compliance with Covenants. The Corporation shall have duly complied with and performed, in all material respects, all covenants and agreements of the Corporation herein which are required to be complied with and performed at or before the Additional Closing.

(d) Certificate of Compliance. The Corporation shall have provided to the Additional Closing Investors a certificate, dated the date of the Additional Closing, in form and substance reasonably satisfactory to the Additional Closing Investors, confirming compliance with the conditions set forth in Sections 6.2(b) and 6.2(c).

(e) Related Agreements and Documents. At or before the Additional Closing, the parties thereto shall have executed and delivered the Transaction Documents.

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(f) Delivery of Certificates for Series E Shares. The Corporation shall have delivered to each Additional Closing Investor a certificate for the Series E Shares being purchased by such Additional Closing Investor at the Additional Closing, registered in the name of such Additional Closing Investor.

(g) Delivery of Warrants. The Corporation shall have executed and delivered to each Additional Closing Investor a Warrant at the Additional Closing, issued in the name of such Additional Closing Investor in accordance with Section 2.4.

SECTION 7. Conditions Precedent to Closing by the Corporation. The obligation of the Corporation to issue and sell the Series E Shares and Warrants being sold to the Investors at any Closing is subject to satisfaction (or the waiver by the Corporation) of the following conditions precedent at or before the Closing:

7.1 Representations and Warranties. The representations and warranties made in Section 5 hereof by the Investors purchasing shares at the Closing shall be true and correct when made, and shall be true and correct in all material respects at the time of the Closing with the same force and effect as if they had been made at and as of the time of the Closing.

7.2 Related Agreements and Documents. At or before the Closing, the parties thereto shall have executed and delivered the Transaction Documents.

7.3 Tender of Payment. At such Closing, the Investors shall have tendered payment for the Series E Shares and Warrants being sold at the Closing.

SECTION 8. Fees; Brokers.

8.1 Fees for Amendments, Etc. The Corporation shall pay, and save the Investors harmless from all liability for the payment of, the reasonable fees and disbursements of any single counsel for the Investors in connection with any modification, amendment or alteration of, or waiver with respect to, this Agreement or the other Transaction Documents, or in connection with enforcement of the Investors’ respective rights hereunder or thereunder.

8.2 Brokers. The Corporation represents and warrants to the Investors that (a) neither the Corporation nor any of its officers, directors, employees or stockholders, has employed any broker or finder in connection with the transactions contemplated by this Agreement, and (b) no person or entity will have, as a result of the transactions contemplated by this Agreement, any right to, interest in, or claim against or upon the Corporation or any Investor for, any commission, fee or other compensation as a finder or broker because of any act or omission by the Corporation or any agent of the Corporation.

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SECTION 9. Remedies. In case any one or more of the representations, warranties, covenants or agreements set forth in this Agreement shall have been breached by the Corporation, the Investors may proceed to protect and enforce their rights either by suit in equity or by action at law, including, but not limited to, an action for damages as a result of any such breach or an action for specific performance of any such covenant or agreement contained in this Agreement.

SECTION 10. Indemnification; Limitations on Liability.

10.1 The Corporation shall indemnify, defend and hold the Investors harmless from and against all liabilities, losses, and damages, together with all reasonable costs and expenses related thereto (including, without limitation, reasonable legal and accounting fees and expenses), which would not have been incurred if (a) all of the representations and warranties of the Corporation in the Transaction Documents (each as modified by any schedule referred to therein) had been true and correct when made and at the time of any Closing, (b) all of the covenants and agreements of the Corporation in the Transaction Documents had been duly and timely complied with and performed and (c) a third party claim against such party had not been brought due to the Corporation not duly and timely complying with the advanced notice provisions of the warrants and agreements listed on Schedule 4.11; provided, however, that the aggregate liability of the Corporation to each Investor under this Section 10.1 shall not exceed the aggregate purchase price of the Series E Shares and the Warrants purchased by such Investor hereunder.

SECTION 11. Exchanges; Lost, Stolen or Mutilated Certificates. Upon surrender by any Investor to the Corporation of or any certificate representing Series E Shares or Reserved Shares, the Corporation at its expense shall issue in exchange therefor, and deliver to such Investor, new certificates representing such Series E Shares or Reserved Shares, as the case may be, in such amounts or denominations as may be requested by such Investor. Upon receipt of evidence satisfactory to the Corporation of the loss, theft, destruction or mutilation of any certificate representing any Series E Shares or Reserved Shares and in case of any such loss, theft or destruction, upon delivery of an indemnity agreement satisfactory to the Corporation, or in case of any such mutilation, upon surrender and cancellation of such certificate, the Corporation at its expense shall issue and deliver to such Investor a new certificate for such Series E Shares or Reserved Shares, of like tenor, in lieu of such lost, stolen or mutilated certificate.

SECTION 12. Survival of Representations, Warranties and Agreements. The covenants, representations and warranties of the parties contained herein shall survive any Closing hereunder. Each of the parties may rely on such covenants, representations and warranties irrespective of any investigation made, or notice or knowledge held by, it or any other person. All statements contained in any certificate or other instrument delivered by any party pursuant to this Agreement or in connection with the transactions contemplated by this Agreement shall constitute representations and warranties by such party under this Agreement, subject to the qualifications set forth herein and therein.

19

SECTION 13. Successors and Assigns. This Agreement shall be binding upon, and inure to the benefit of, each of the parties hereto and, except as otherwise expressly provided herein, each other person who shall become a registered holder named in a certificate evidencing Series E Shares, Warrants or Reserved Shares transferred to such holder by any of the Investors or their permitted transferees, and (except as aforesaid) their respective legal representatives, successors and assigns.

SECTION 14. Entire Agreement; Effect on Prior Documents. This Agreement and the other documents referred to herein or delivered pursuant hereto contain the entire agreement among the parties with respect to the financing transactions contemplated hereby and supersede all prior negotiations, commitments, agreements and understandings among them with respect thereto.

SECTION 15. Notices. All notices, requests, consents and other communications hereunder (“Notices”) to any party shall be contained in a written instrument addressed to such party at the address set forth below or such other address as may hereafter be designated in writing by the addressee to the addressor listing all parties and shall be deemed given (a) when delivered in person or duly sent by fax showing confirmation of receipt, (b) three days after being duly sent by first class mail postage prepaid (other than in the case of Notices to or from any non-U.S. resident, which Notices must be sent in the manner specified in clause (a) or (c)), or (c) two days after being duly sent by DHL, Federal Express or other recognized express international courier service:

(a) if to the Corporation, to:

Acceleron Pharma Inc.

128 Sidney Street

Cambridge, MA 02139

with a copy to:

Ropes & Gray LLP

One International Place

Boston, MA 02110

Attn: Marc Rubenstein

Fax: (617) 951-7050

(b) if to the Investors, to their respective addresses as set forth on the Schedule of Investors.

SECTION 16. Amendments; Waivers. This Agreement may be amended, and compliance with the provisions of this Agreement may be omitted or waived, only by the written agreement of the Corporation and Investors or assignees of their rights hereunder holding at least seventy-five percent (75%) in voting power of the then outstanding Series E Shares and Reserved Shares issued upon conversion thereof, provided, however, that no Investor shall, without its consent, be adversely affected by any such modification, amendment or waiver in any manner in which the other Investors are not likewise adversely affected.

20

SECTION 17. Counterparts. This Agreement may be executed in any number of counterparts, each such counterpart shall be deemed to be an original instrument, and all such counterparts together shall constitute but one agreement. Any such counterpart may contain one or more signature pages.

SECTION 18. Headings. The headings of the various sections of this Agreement have been inserted for convenience of reference only and shall not be deemed to be a part of this Agreement.

SECTION 19. Nouns and Pronouns. Whenever the context may require, any pronouns used herein shall include the corresponding masculine, feminine or neuter forms, and the singular form of names and pronouns shall include the plural and vice-versa.

SECTION 20. Governing Law. This Agreement shall be governed by, and construed and enforced in accordance with, the substantive laws of the Commonwealth of Massachusetts, without regard to its principles of conflicts of laws.

SECTION 21. Severability. Any provision of this Agreement that is prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction, be ineffective to the extent of such prohibition or unenforceability without invalidating the remaining provisions hereof, and any such prohibition or unenforceability in any jurisdiction shall not invalidate or render unenforceable such provision in any other jurisdiction.

21

IN WITNESS WHEREOF, the undersigned have executed this Series E Convertible Preferred Stock and Warrant Purchase Agreement as of the day and year first written above.

ACCELERON PHARMA INC.
By:	/s/ John Knopf
	John Knopf, Chief Executive Officer

[ Company Signature Page for Stock Purchase Agreement ]

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 10^th day of June, 2010.

ADVANCED TECHNOLOGY VENTURES VII, LP		ADVANCED TECHNOLOGY VENTURES VI, LP
By:	ATV Associates VII, L.L.C., Its General Partner	By:	ATV Associates VI, L.L.C., Its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
Address:	Bay Colony Corporate Center 1000 Winter Street, Suite 3700 Waltham, MA 02451	Address:	Bay Colony Corporate Center 1000 Winter Street, Suite 3700 Waltham, MA 02451
ADVANCED TECHNOLOGY VENTURES VII (B), LP		ADVANCED TECHNOLOGY VENTURES VII (C), LP
By:	ATV Associates VII, L.L.C., Its General Partner	By:	ATV Associates VII, L.L.C., Its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
Address:	Bay Colony Corporate Center 1000 Winter Street, Suite 3700 Waltham, MA 02451	Address:	Bay Colony Corporate Center 1000 Winter Street, Suite 3700 Waltham, MA 02451
ATV ENTREPRENEURS VI, LP		ATV ENTREPRENEURS VII, LP
By:	ATV Associates VI, L.L.C., Its General Partner	By:	ATV Associates VII, L.L.C., Its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
Address:	Bay Colony Corporate Center 1000 Winter Street, Suite 3700 Waltham, MA 02451	Address:	Bay Colony Corporate Center 1000 Winter Street, Suite 3700 Waltham, MA 02451

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 10^th day of June, 2010.

BESSEMER VENTURE PARTNERS VII L.P.
BESSEMER VENTURE PARTNERS VII INSTITUTIONAL L.P.
By:	Deer VII & Co. L.P., their General Partner
By:	Deer VII & Co. Ltd., its General Partner

By:	/s/ J. Edmund Colloton
	Name: J. Edmund Colloton
	Title: Director

Address: c/o Bessemer Venture Partners 1865 Palmer Avenue, Suite 104 Larchmont, NY 10538

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 10^th day of June, 2010.

CADUCEUS PRIVATE INVESTMENTS II, LP		CADUCEUS PRIVATE INVESTMENTS II (QP), LP
By:	OrbiMed Capital GP II LLC Its General Partner	By:	OrbiMed Capital GP II LLC Its General Partner
By:	/s/ Carl Gordon	By:	/s/ Carl Gordon
Name:	Carl Gordon	Name:	Carl Gordon
Title:	General Partner	Title:	General Partner
Address:	767 Third Avenue 30th Floor New York, NY 10017	Address:	767 Third Avenue 30th Floor New York, NY 10017
UBS JUNIPER CROSSOVER FUND, L.L.C.
By:	OrbiMed Advisors LLC Its Member
By:	/s/ Carl Gordon
Name:	Carl Gordon
Title:	General Partner
Address:	767 Third Avenue 30th Floor New York, NY 10017

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 10^th day of June, 2010.

POLARIS VENTURE PARTNERS IV, L.P.		POLARIS VENTURE PARTNERS ENTREPRENEURS’ FUND IV, L.P.
By:	Polaris Venture Management Co. IV, L.L.C. Its General Partner	By:	Polaris Venture Management Co. IV, L.L.C. Its General Partner
By:	/s/ William E. Bilodeau	By:	/s/ William E. Bilodeau
Name:	William E. Bilodeau	Name:	William E. Bilodeau
Title:	Attorney-in-fact	Title:	Attorney-in-fact
Address:	1000 Winter Street, Suite 3350 Waltham, MA 02451	Address:	1000 Winter Street, Suite 3350 Waltham, MA 02451

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 10^th day of June, 2010.

QVT Fund LP		Quintessence Fund L.P.
By:	Its General Partner, QVT Associates GP LLC	By:	Its General Partner, QVT Associates GP LLC

By:	/s/ Keith Manchester	By:	/s/ Keith Manchester
Name:	Keith Manchester	Name:	Keith Manchester
Title:	Authorized Signatory	Title:	Authorized Signatory
Address:	c/o QVT Financial LP 1177 Avenue of the Americas 9th Floor New York, NY 10036	Address:	c/o QVT Financial LP 1177 Avenue of the Americas 9th Floor New York, NY 10036

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 10^th day of June, 2010.

SUTTER HILL VENTURES, A CALIFORNIA LIMITED PARTNERSHIP		G. LEONARD BAKER, JR. AND MARY ANNE BAKER, CO-TRUSTEES OF THE BAKER REVOCABLE TRUST U/A/D 2/3/03
By: Its:	Sutter Hill Ventures, L.L.C. General Partner
		By:	/s/ Robert Yin
By:	/s/ Jeffrey W. Bird		G. Leonard Baker Jr. Trustee
Name:	Jeffrey W. Bird		By Robert Yin
Title:	Managing Director		Under Power of Attorney
SAUNDERS HOLDINGS, L.P.		JEFFREY W. BIRD AND CHRISTINA R. BIRD AS TRUSTEES OF JEFFREY W. AND CHRISTINA R. BIRD TRUST AGREEMENT DATED 10/31/00
By:	/s/ Robert Yin
	G. Leonard Baker Jr. Trustee of The Baker	By:	/s/ Jeffrey W. Bird
	Revocable Trust U/A/D 2/3/03 General Partner		Jeffrey W. Bird, Trustee
	By Robert Yin
	Under Power of Attorney
GREGORY P. SANDS AND SARAH J.D. SANDS AS TRUSTEES OF GREGORY P AND SARAH J.D. SANDS TRUST AGREEMENT DATED 2/24/99		DAVID L. ANDERSON, TRUSTEE OF THE ANDERSON LIVING TRUST U/A/D 1/22/98
By:	/s/ Robert Yin	By:	/s/ David L. Anderson
	Gregory P. Sands, Trustee		David L. Anderson, Trustee
	By Robert Yin
	Under Power of Attorney
ANVEST, L.P.		YOVEST, L.P.
By:	/s/ David L. Anderson	By:	/s/ William H. Younger
	David L. Anderson, Trustee of The Anderson Living Trust U/A/D 1/22/98, General Partner		William H. Younger, Jr., Trustee of The William H. Younger, Jr. Revocable Trust U/A/D 8/5/09, General Partner

Signature Page to Stock Purchase Agreement

JAMES C. GAITHER TRUSTEE OF THE GAITHER REVOCABLE TRUSTEE U/A/D 9/28/00		TALLACK PARTNERS, L.P.
		By:	/s/ Robert Yin
By:	/s/ Robert Yin		James C. Gaither, Trustee of The Gaither Revocable Trust U/A/D 9/28/2000, General Partner
	James C. Gaither, Trustee
	By Robert Yin		By Robert Yin
	Under Power of Attorney		Under Power of Attorney
JAMES N. WHITE AND PATRICIA A. O’BRIEN AS TRUSTEES OF THE WHITE FAMILY TRUST U/A/D 4/3/97
By:	/s/ Robert Yin
	James N. White, Trustee
	By Robert Yin
	Under Power of Attorney

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 10^th day of June, 2010.

VENROCK PARTNERS, L.P.,

By its General Partner, Venrock Partners Management, LLC

VENROCK ASSOCIATES IV, L.P.,

By its General Partner, Venrock Management IV, LLC

VENROCK ENTREPRENEURS FUND IV, L.P.

By its General Partner, VEF Management IV, LLC

By:	/s/ Anthony B. Evnin
	Name: Anthony B. Evnin
	Title: Member

Address:	530 Fifth Avenue, 22nd Floor
	New York, NY 10036

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 10^th day of June, 2010.

Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Tench Coxe	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Robert Yin
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Andrew T. Sheehan	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO David E. Sweet (Rollover)
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Lynne B. Graw	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Diane J. Naar
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Yu-Ying Chen	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Patricia Tom (Post)
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Sherryl W. Casella	Address: Wells Fargo Bank, N.A. Attention: Vicki Bandel 600 California Street 12th Floor MAC A0193-120 San Francisco, CA 94108 Phone (415) 396-3739 Fax (415) 975-7539 Email: bandel@wellsfargo.com The above WFB address is for overnite mail (Fed Ex. etc.) For mailing, the address is Ms. Vicki Bandel Wells Fargo Bank MAC-A0101-021 P.O. Box 63050 San Francisco, CA 94163

BY:	/s/ VICKI M. BANDEL
Name:	VICKI M. BANDEL
Title:	ASSISTANT VICE PRESIDENT TRUST OFFICER

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

ALKERMES, INC.
By:	/s/ JAMES M. FRATES
Name:	JAMES M. FRATES
Title:	SVP, CFO AND TREASURER
Address:	852 Winter Street Waltham, MA 02451

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

CELGENE CORPORATION
By:	/s/ Robert J. Hugin
Name:	Robert J. Hugin
Title:	CEO
Address:	86 Morris Avenue Summit, NJ 07901

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

Hercules Technology II, L.P.
By:	/s/ K. Nicholas Martitsch
Name:	K. Nicholas Martitsch
Title:	Associate General Counsel
Address:	400 Hamilton Avenue Suite 310 Palo Alto, CA 94301

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

/s/ Wylie Vale
Wylie Vale
Address:	1643 Valdes Drive La Jolla, CA 92037

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

ROPART INVESTMENTS LLC
By:	/s/ Robert B. Goergen
Name:	Robert B. Goergen
Title:	Managing Member

Address:	Attn: Peter Cawley
	One East Weaver Street
	Greenwich, CT 06831

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

THE KONRAD HANS VON EMSTER III AND ELIZABETH F. VON EMSTER REVOCABLE TRUST DATED JANUARY 18, 2005
By:	/s/ Konrad Von Emster III	Elizabeth F. Von Emster
Name:	Konrad Von Emster III	Elizabeth F. Von Emster
Title:	Trustee	Trustee

Address:	1647 Ralston Avenue
	Belmont, CA 94002

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

NEXT CHAPTER HOLDINGS LP
By:	/s/ Mark R. Pattis
Name:	Mark R. Pattis
Title:	President, Corp G.P.

Address:	c/o Mark R. Pattis
	600 Central Avenue,
	Suite 205-210
	Highland Park, IL 80035

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

/s/ Vaughn Kailian
Vaughn Kailian
Address:	1100 Fitzpatrick Lane
	P.O. Box 70 Bodega, CA 94922

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

/s/ Leon Smith
Leon Smith
Address:	39 Holton Lane
	Essex Falls, NJ 07021

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series E Convertible Preferred Stock and Warrant Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated as of June 10, 2010 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 9 day of July, 2010.

MIDCAP FINANCIAL, LLC, a Delaware limited liability company
By:	/s/ Josh Groman
Name:	Josh Groman
Title:	Managing Director
Address:	7735 Old Georgetown Road
	Suite 400 Bethesda, MD 20814 Attn: Mr. Bob Goodridge

Signature Page to Stock Purchase Agreement

EX-5

Exhibit 5

SERIES F CONVERTIBLE PREFERRED STOCK

PURCHASE AGREEMENT

THIS SERIES F CONVERTIBLE PREFERRED STOCK PURCHASE AGREEMENT (“Agreement”) is made as of December 22, 2011 by and among Acceleron Pharma Inc., a Delaware corporation (the “Corporation”) and the investors that are named on the Schedule of Investors attached hereto (the “Investors”).

WHEREAS, the Investors wish to purchase from the Corporation, and the Corporation wishes to sell to the Investors, up to an aggregate of 9,704,756 shares of the Corporation’s Series F Convertible Preferred Stock, $.001 par value per share (the “Series F Preferred Stock”);

NOW, THEREFORE, in consideration of the premises and the mutual covenants and agreements herein contained, the parties hereby agree as follows:

SECTION 1. Amended and Restated Certificate of Incorporation. Prior to the Closing (as defined in Section 3.1), the Corporation filed with the Secretary of State of the State of Delaware an Amended and Restated Certificate of Incorporation (the Certificate of Incorporation of the Corporation as so amended and restated and as in effect on the date hereof being hereinafter referred to as the “Certificate of Incorporation”), a copy of which is attached hereto as Exhibit 1.

SECTION 2. Purchase and Sale of the Series F Preferred Stock.

2.1 Series F Shares. Subject to the terms and conditions of this Agreement, at the Closing, the Corporation agrees to issue and sell to the Investors identified on the Schedule of Investors the number of shares of Series F Preferred Stock (the “Series F Shares”) set forth on the Schedule of Investors. Each Investor named on the Schedule of Investors, acting severally and not jointly, agrees to purchase from the Corporation at the Closing the number of Series F Shares, at the aggregate purchase price, set forth opposite the name of such Investor on the Schedule of Investors.

SECTION 3. Closing.

3.1 Closing. The closing of the sale and purchase of the Series F Shares (the “Closing”) shall take place upon the execution of this Agreement at the offices of Ropes & Gray LLP, Prudential Tower, 800 Boylston Street, Boston, Massachusetts, or at such other location as may be agreed upon among the Investors and the Corporation. At the Closing, the Corporation shall issue and deliver to each Investor a certificate or certificates for shares of Series F Preferred Stock, registered in the name of such Investor, in the amount representing the number of Series F Shares being purchased by such Investor, against payment by such Investor to the Corporation of the aggregate purchase price therefor in the form of (a) a certified or bank check payable to the order of the Corporation, (b) a wire transfer to a bank account designated by the Corporation or (c) a combination of (a) and (b).

SECTION 4. Representations and Warranties of the Corporation. The Corporation represents and warrants to the Investors as follows:

4.1 Organization. The Corporation is a corporation duly organized, validly existing and in good standing under the laws of the State of Delaware and has all requisite corporate power and authority to own and lease its properties, to carry on its business as presently conducted and as proposed to be conducted by it and to carry out the transactions contemplated by this Agreement. The Corporation is duly qualified as a foreign corporation and is in good standing in all such jurisdictions (which jurisdictions are listed in Schedule 4.1A) in which the conduct of its business or its ownership or leasing of property requires such qualification, except to the extent that any failure to be so qualified would not materially and adversely affect the financial condition, results of operations, assets, liabilities business or prospects of the Corporation.

4.2 Capitalization. Immediately prior to the Closing, the entire authorized capital stock of the Corporation consists of:

(a) 104,013,161 shares of Common Stock, $.001 par value per share (“Common Stock”), of which (i) 9,579,636 shares have been issued and are outstanding, fully paid and nonassessable; (ii) no shares are held as treasury shares; (iii) 18,000,000 have been reserved for issuance upon exercise of options granted or to be granted under stock purchase, stock option or other equity incentive plans of the Corporation; (iv) 26,069,980 shares have been reserved for issuance upon conversion of the outstanding shares of Series A Preferred Stock of the Corporation and shares of Series A Preferred Stock issuable upon exercise of outstanding warrants; (v) 16,944,378 shares have been reserved for issuance upon conversion of the Series B Preferred Stock and shares of Series B Preferred Stock issuable upon exercise of outstanding warrants; (vi) 11,923,077 shares have been reserved for issuance upon conversion of the Series C Preferred Stock; (vii) 2,014,652 shares have been reserved for issuance upon conversion of the Series C-1 Preferred Stock and shares of Series C-1 Preferred Stock issuable upon exercise of outstanding warrants; (viii) 955,414 shares have been reserved for issuance upon conversion of the Series D Preferred Stock; (ix) 2,802,548 shares have been reserved for issuance upon conversion of the Series D-1 Preferred Stock; (x) 3,662,422 shares have been reserved for issuance upon conversion of the Series E Preferred Stock and 3,486,395 shares have been reserved for issuance upon exercise of outstanding warrants issued pursuant to the Series E Convertible Preferred Stock and Warrant Purchase Agreement dated June 10, 2010; and (xi) 9,704,756 shares have been reserved for issuance upon conversion of the Series F Preferred Stock;

(b) 26,069,980 of Series A Preferred Stock, 25,643,980 shares of which are issued and outstanding;

(c) 16,944,378 shares of Series B Preferred Stock, 16,816,810 of which are issued and outstanding;

(d) 11,923,077 shares of Series C Preferred Stock, 11,912,333 shares of which are issued and outstanding;

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(e) 2,014,652 shares of Series C-1 Preferred Stock, 1,831,502 shares of which are issued and outstanding;

(f) 955,414 shares of Series D Preferred Stock, 939,847 of which are issued and outstanding;

(g) 2,802,548 shares of Series D-1 Preferred Stock, 2,547,771 of which are issued and outstanding;

(h) 3,662,422 shares of Series E Preferred Stock, 3,264,333 of which are issued and outstanding; and

(i) 9,704,756 shares of Series F Preferred Stock, all of which are being issued at the Closing and immediately thereafter will be issued and outstanding, fully paid and nonassessable and will be held by the Investors.

Schedule 4.2 contains a list of all holders of capital stock of the Corporation and options, warrants or rights to purchase such capital stock that will be outstanding immediately before the Closing, in each case including the number of shares of capital stock held by, or subject to purchase pursuant to the exercise of any option, warrant or right held by, each such holder. Except as set forth in Schedule 4.2, there are no outstanding shares of capital stock of the Corporation or warrants, options, agreements, convertible securities or other commitments pursuant to which the Corporation is or may become obligated to issue any shares of its capital stock or other securities as of the Closing. Except as set forth in Schedule 4.2, the number of shares of capital stock, if any, issuable in connection with the securities described in the first sentence of this paragraph is not subject to adjustment by reason of the issuance of the Series F Shares or the Reserved Shares (as defined in Section 4.22). There are no preemptive or similar rights to purchase or otherwise acquire shares of capital stock of the Corporation from the Corporation pursuant to any provision of law, the Certificate of Incorporation or the By-Laws or, except as set forth in Schedule 4.2, any agreement to which the Corporation is a party, or otherwise. Except as set forth in the Amended and Restated Registration Rights Agreement, Amended and Restated Voting Agreement, Amended and Restated Right of First Refusal and Co-Sale Agreement and Amended and Restated Investor Rights Agreement (each as defined in Section 4.22) or in Schedule 4.2, there is no agreement, restriction or encumbrance with respect to the sale or voting of any shares of the Corporation’s capital stock (whether outstanding or issuable upon conversion or exercise of outstanding securities). The Corporation has not violated the Securities Act of 1933, as amended (the “Securities Act”) or any securities law of any state or other jurisdiction in connection with the issuance of any securities.

4.3 Equity Investments. Except as set forth in Schedule 4.3, the Corporation does not currently own, directly or indirectly, any capital stock or other proprietary interest in any corporation, association, trust, partnership, limited liability company, limited liability partnership, joint venture or other entity.

4.4 Financial Statements. Attached as Schedule 4.4 is the unaudited consolidated balance sheet of the Corporation (the “Balance Sheet”) as of September 30, 2011 (the “Balance Sheet Date”). The Balance Sheet (a) is true and correct in all material respects, (b) is in accordance with the books and records of the Corporation, and (c) presents fairly the financial position of the Corporation as of the Balance Sheet Date.

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4.5 Absence of Undisclosed Liabilities. Except as set forth in Schedule 4.5 or as reflected on the Balance Sheet, the Corporation has no liabilities of any nature (matured or unmatured, fixed or contingent) that individually or in the aggregate exceed $50,000.

4.6 Absence of Changes. Except as set forth in Schedule 4.6, since the Balance Sheet Date there has not been (a) any change in the financial condition, results of operations, assets, liabilities, business or prospects of the Corporation that, individually or in the aggregate, is materially adverse, (b) any material asset or property of the Corporation made subject to a lien of any kind, except liens for taxes not yet due and payable, (c) any satisfaction or discharge of any lien, claim, or encumbrance or payment of any obligation by the Corporation, except in the ordinary course of business and that is not material to the financial condition, results of operations, assets, liabilities, business or prospects of the Corporation, (d) any waiver of any valuable right of the Corporation, or the cancellation of any debt or claim held by the Corporation, (e) any payment of dividends on, or other distribution with respect to, or any direct or indirect redemption or acquisition of, any shares of the capital stock of the Corporation, or any agreement or commitment therefor, (f) any mortgage, pledge, sale, assignment or transfer of any tangible or intangible assets of the Corporation, except in the ordinary course of business, (g) any loan by the Corporation to, or any loan to the Corporation from, any officer, director, employee or stockholder of the Corporation, or any agreement or commitment therefor, (h) any damage, destruction or loss (whether or not covered by insurance) materially and adversely affecting the assets, property or business of the Corporation, (i) any change in the accounting methods or practices followed by the Corporation, (j) any amendment to the Corporation’s Certificate of Incorporation or Bylaws, except as provided in this Agreement or (k) any agreement or commitment by the Corporation to do any of the things described in this Section 4.6. Each of the occurrences described in clauses (a) through (k) above is sometimes hereinafter referred to as a “Material Adverse Change.”

4.7 Encumbrances. The Corporation has good and marketable title to all of its property and assets, real, personal or mixed, tangible or intangible, free and clear of all liens, security interests, charges and other encumbrances of any kind, except (i) liens for taxes not yet due and payable, (ii) liens securing debt reflected on the Balance Sheet, and (iii) liens arising in the ordinary course of business.

4.8 Permits. Except as set forth in Schedule 4.8, the Corporation has all franchises, permits, licenses, authorizations, approvals, certificates, consents, orders and any similar authority (collectively, the “Permits”) from state, federal and regulatory authorities of the United States, including without limitation, the United States Food and Drug Administration (the “FDA”), necessary for the conduct of its business as now being conducted by it, except for those the lack of which would not reasonably be expected to cause a Material Adverse Change. All such Permits are valid and in full force and effect. There is no FDA enforcement action pending or, to the knowledge of the Corporation, threatened against the Corporation. The Corporation is not in default in any material respect under any of such franchises, permits, licenses or other similar authority.

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4.9 Intellectual Property Rights.

(a) All of the patents, trademarks, service marks, trade names, copyrights, applications for patents, trademarks, service marks and copyrights owned or licensed exclusively by the Corporation are listed in Schedule 4.9 (the “Scheduled Intellectual Property”);

(i) Except as set forth in Schedule 4.9, no royalties or other amounts are payable by the Corporation to other persons by reason of the ownership or use of the Scheduled Intellectual Property or the other Intellectual Property Rights (as defined below);

(ii) To the knowledge of the Corporation and except as set forth on Schedule 4.9, (i) no product or service marketed or sold by the Corporation and no Intellectual Property Rights licensed or proposed to be licensed by the Corporation as licensor violates any license or infringes any Intellectual Property Rights of another, and (ii) the Corporation has not received any notice that any of the Intellectual Property Rights identified in Schedule 4.9 or the operation of the business of the Corporation conflicts or will conflict with the Intellectual Property Rights of others; and

(b) There are no claims pending or threatened with respect to any Intellectual Property Rights necessary or required for the conduct of the business of the Corporation as currently conducted, nor, to the Corporation’s knowledge, does there exist any basis therefor.

As used herein, the term “Intellectual Property Rights” means all patents, trademarks, service marks, trade names, copyrights, inventions, trade secrets, know-how, proprietary processes and formulae, applications for patents, trademarks, service marks and copyrights, and other industrial and intellectual property rights.

4.10 Litigation. Except as set forth in Schedule 4.10, there is no action, suit, claim, proceeding or investigation, at law, in equity or otherwise, or by or before any governmental instrumentality or other agency, now pending or, to the knowledge of the Corporation, threatened against or affecting the Corporation, nor, to the knowledge of the Corporation, does there exist any basis therefor.

4.11 No Defaults. Except as set forth in Schedule 4.11, the Corporation is not in violation or breach of, or in default under, any provision of the Certificate of Incorporation or By-Laws, and there exists no condition, event or act which after notice, lapse of time, or both, may constitute a violation or breach of, or a default under, the Certificate of Incorporation or By-Laws. The Corporation is not in violation or breach of, or in default under any note, indenture, mortgage, lease, contract, purchase order or other instrument, document or agreement to which the Corporation is a party or by which it or any of its property is bound or affected or any ruling, writ, injunction, order, judgment or decree of any court, administrative agency or other governmental body, except to the extent that any of the foregoing individually or in the aggregate would not result in a Material Adverse Change. Except as set forth in Schedule 4.11, to the knowledge of the Corporation, there exists no condition, event or act which after notice, lapse of time, or both, may constitute a violation or breach of, or a default under, any of the foregoing.

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4.12 Employment of Founder, Officers, Employees and Consultants. To the knowledge of the Corporation and except as set forth on Schedule 4.12, no employee of the Corporation is in violation of any term of any contract or covenant (either with the Corporation or with another entity) relating to employment, consulting, assignment of inventions, proprietary information disclosure, non-competition or non-solicitation. To the knowledge of the Corporation and except as set forth on Schedule 4.12, John Knopf (the “Founder”) is not obligated under any contract or subject to any judgment, decree or administrative order that would conflict or interfere with (i) the performance of the Founder’s duties as an employee, consultant, director or officer of the Corporation, as the case may be, or (ii) the Corporation’s business as conducted or proposed to be conducted.

4.13 Taxes. The Corporation has filed all federal, state, local and foreign tax returns which are required to be filed by it and all such returns are true and correct in all material respects. The Corporation has paid all taxes shown as due thereon, or pursuant to any assessments received by it or which it is obligated to withhold from amounts owing to any employee, creditor or third party, except, in each case, for those taxes which are not yet due and payable pursuant to such returns.

4.14 Material Agreements. Except as set forth in Schedule 4.14, the Corporation is not a party to any written or oral (a) contract with any labor union; (b) contract for the future purchase of fixed assets or materials, supplies or equipment in excess of normal operating requirements; (c) contract for the employment of any officer, employee or other person or any contract with any person on a consulting basis; (d) bonus, pension, profit-sharing, retirement, stock purchase, stock option, hospitalization, medical insurance or similar plan, contract or understanding in effect with respect to employees or any of them or the employees of others; (e) agreement or indenture relating to the borrowing of money or to the mortgaging, pledging or otherwise placing a lien on any assets of the Corporation; (f) guaranty of any obligation for borrowed money or otherwise; (g) lease or agreement under which the Corporation is lessee of or holds or operates any property, real or personal, owned by any other party; (h) lease or agreement under which the Corporation is lessor of or permits any third party to hold or operate any property, real or personal, owned or controlled by the Corporation; (i) license or lease agreement with respect to any Intellectual Property Rights; (j) agreement or other commitment for capital expenditures in excess of $50,000; (k) contract, agreement or commitment under which the Corporation is obligated to pay any broker’s fees, finder’s fees or any such similar fees, to any third party; or (l) any other contract, agreement, arrangement or understanding which is material to the business of the Corporation. Each contract, agreement, arrangement, understanding or commitment listed on Schedule 4.14 (each a “Material Agreement”) is a valid and binding obligation of the Corporation, and, to the knowledge of the Corporation, each other party thereto, enforceable against the Corporation in accordance with its terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)), and has been duly executed and delivered by the Corporation and, to the knowledge of the Corporation, each other party thereto. With respect to each Material Agreement, (a) the Corporation and, to the knowledge of the Corporation, each other party to such agreement has performed in all material respects all obligations required to be performed to date under such Material Agreement; (b) to the knowledge of the Corporation, no party to such Material Agreement is in default or arrears under the terms of such Material Agreement; and (c) to the knowledge of the Corporation, no condition exists or event has occurred that, with the

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giving of notice or lapse of time or both, would constitute a default under such Material Agreement. The Corporation has furnished to the Investors true and correct copies of all such written agreements and other documents, in each case to the extent requested by the Investors or their authorized representatives.

4.15 ERISA. Except as set forth on Schedule 4.15, neither the Corporation nor any entity required to be aggregated with the Corporation under Sections 414(b), (c), (m) or (n) of the Internal Revenue Code of 1986, as amended (the “Code”), sponsors, maintains, has any obligation to contribute to, has any liability under, or is otherwise a party to, any Benefit Plan. For purposes of this Agreement, “Benefit Plan” shall mean any plan, fund, program, policy, arrangement or contract, whether formal or informal, which is in the nature of (i) an employee pension benefit plan (as defined in Section 3(2) of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”)) or (ii) an employee welfare benefit plan (as defined in section 3(1) of ERISA).

4.16 U.S. Real Property Holding Corporation. The Corporation is not now and has never been a “United States real property holding corporation,” as defined in Section 897(c)(2) of the Code and Section 1.897-2(b) of the Treasury Regulations promulgated by the Internal Revenue Service, and the Corporation has never filed with the Internal Revenue Service a statement with its United States income tax returns under Section 1.897-2(h) of such Regulations stating that any shares of its capital stock constitute a U.S. real property interest within the meaning of Section 897(c)(1) of the Code.

4.17 Environmental Protection. Except as set forth on Schedule 4.17, the Corporation has not caused or allowed, or contracted with any party for, the generation, use, transportation, treatment, storage or disposal of any Hazardous Substances (as defined below) in connection with the operation of its business or otherwise. The Corporation does not presently own any real property. The operation of the business of the Corporation is in compliance with all applicable Environmental Laws (as defined below) and orders or directives of any governmental authorities having jurisdiction under such Environmental Laws, including, without limitation, any Environmental Laws or orders or directives with respect to any cleanup or remediation of any release or threat of release of Hazardous Substances. The Corporation has not received any citation, directive, letter or other communication, written or oral, or any notice of any proceeding, claim or lawsuit, from any person arising out of the conduct of its operations, and the Corporation is not aware of any basis therefor. The Corporation has obtained and is maintaining in full force and effect all necessary permits, licenses and approvals required by all Environmental Laws applicable to the business operations conducted by the Corporation, and is in compliance with all such permits, licenses and approvals. For the purposes of this Agreement, the term “Environmental Laws” shall mean any Federal, state or local law or ordinance or regulation pertaining to the protection of human health or the environment, including, without limitation, the Comprehensive Environmental Response, Compensation, and Liability Act, 42 U.S.C. Sections 9601, et seq., the Emergency Planning and Community Right-to-Know Act, 42 U.S.C. Sections 11001, et seq., and the Resource Conservation and Recovery Act, 42 U.S.C. Sections 6901 et seq. For purposes of this Agreement, the term “Hazardous Substances” shall include oil and petroleum products, asbestos, polychlorinated biphenyls, urea formaldehyde and other materials classified as hazardous or toxic under any Environmental Laws.

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4.18 Foreign Corrupt Practices Act. The Corporation has not taken any action which would cause it to be in violation of the Foreign Corrupt Practices Act of 1977, as amended, or any rules and regulations thereunder. There is not now, and there has never been, any employment by the Corporation of, or beneficial ownership in the Corporation by, any governmental or political official in any country in the world.

4.19 Federal Reserve Regulations. The Corporation is not engaged in the business of extending credit for the purpose of purchasing or carrying margin securities (within the meaning of Regulation U of the Board of Governors of the Federal Reserve System), and no part of the proceeds of the sale of Series F Shares will be used to purchase or carry any margin security or to extend credit to others for the purpose of purchasing or carrying any margin security or in any other manner which would involve a violation of any of the regulations of the Board of Governors of the Federal Reserve System.

4.20 Compliance. The Corporation has complied in all material respects with all federal, state, local and foreign laws applicable to its business. The Corporation has all federal, state, local and foreign governmental licenses, registrations and permits necessary for the conduct of its business, except for those the absence of which would not materially and adversely affect the financial condition, results of operations, assets, liabilities business or prospects of the Corporation; such licenses, registrations and permits are in full force and effect; and there have been no violations of any such licenses, registrations or permits except for violations that individually or in the aggregate would not result in a Material Adverse Change. Except as set forth on Schedule 4.20, no proceeding is pending, nor has the Corporation received any oral or written notice of any proceeding that is threatened, to revoke or limit any of the foregoing licenses, registrations or permits. Without limiting the generality of the foregoing, the Corporation has complied in all material respects with all applicable laws relating to the employment of labor, including provisions relating to wages, hours, equal opportunity, collective bargaining and the payment of Social Security and other taxes, and with ERISA.

4.21 Insurance. Schedule 4.21 sets forth each insurance policy (specifying the insurer, the amount of coverage, the type of insurance, the policy number, the expiration date, the annual premium and any pending claims thereunder) maintained by the Corporation on its properties, assets, business or personnel. No notice from any insurance carrier has been received by the Corporation claiming that the Corporation is in default with respect to any provision contained in any insurance policy listed on Schedule 4.21.

4.22 Authorization of Transaction Documents. The execution, delivery and performance by the Corporation of (a) this Agreement, (b) the Amended and Restated Registration Rights Agreement of even date herewith by and among the Corporation and the Investors and certain other parties in the form of Exhibit 4.22A (the “Amended and Restated Registration Rights Agreement”), (c) the Amended and Restated Voting Agreement of even date herewith by and among the Corporation and the other parties thereto in the form of Exhibit 4.22B (the “Amended and Restated Voting Agreement”), (d) the Amended and Restated Right of First Refusal and Co-Sale Agreement of even date herewith by and among the Corporation and the other parties thereto in the form of Exhibit 4.22C (the “Amended and Restated Right of First Refusal and Co-Sale Agreement”), and (e) the Amended and Restated Investor Rights Agreement of even date herewith by and among the Corporation and the other parties thereto in

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the form of Exhibit 4.22D (the “Amended and Restated Investor Rights Agreement” and, together with this Agreement, the Amended and Restated Registration Rights Agreement, the Amended and Restated Voting Agreement, the Amended and Restated Right of First Refusal and Co-Sale Agreement and the Amended and Restated Investor Rights Agreement, the “Transaction Documents”) have been duly authorized by all requisite corporate action. The Corporation has duly authorized, executed and delivered each Transaction Document, and each Transaction Document constitutes the valid and binding obligation of the Corporation, enforceable in accordance with its terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)). The execution, delivery and performance of the Transaction Documents, the issuance, sale and delivery of the Series F Shares and the shares of Common Stock issuable upon conversion of the Series F Shares (the “Reserved Shares”), and compliance with the provisions hereof and thereof by the Corporation do not and will not, with or without the passage of time or the giving of notice or both, violate, conflict with or result in any breach of any of the terms, conditions or provisions of, or constitute a default (or give rise to any right of termination, cancellation or acceleration) under, or result in the creation of any lien, security interest, charge or encumbrance upon any of the properties or assets of the Corporation under, the Certificate of Incorporation or Bylaws, any Material Agreement, or any provision of law, statute, rule or regulation or any ruling, writ, injunction, order, judgment or decree of any court naming the Corporation, or administrative agency or other governmental body.

4.23 Authorization of Series F Shares and Reserved Shares. The issuance, sale and delivery hereunder by the Corporation of the Series F Shares have been duly authorized by all requisite corporate action of the Corporation, and when so issued, sold and delivered the Series F Shares will be validly issued and outstanding, fully paid and nonassessable, and not subject to preemptive or any other similar rights of the stockholders of the Corporation or others. The issuance and delivery of the Reserved Shares have been duly authorized by all requisite corporate action of the Corporation, and the Reserved Shares have been duly reserved for issuance upon conversion of any or all of the Series F Shares, and when so issued and delivered upon conversion of the Series F Shares, the Reserved Shares will be validly issued and outstanding, fully paid and nonassessable, and not subject to preemptive or any other similar rights of the stockholders of the Corporation or others.

4.24 Related Transactions. Except as set forth in Schedule 4.24, no director, officer or employee of the Corporation nor any “associate” (as defined in Rule 12b-2 promulgated under the Securities Exchange Act of 1934, as amended (the “Exchange Act”)) of any such person is indebted to the Corporation, nor is the Corporation indebted (or committed to make loans or extend or guarantee credit) to any such person, nor is any such person a party to any transaction (other than as an employee or consultant) with the Corporation providing for the furnishing of services by, or rental of real or personal property from, or otherwise requiring cash payments to, any such person.

4.25 Offerees. Except as set forth in Schedule 4.25, during the six months prior to the date of this Agreement, the Corporation has not, either directly or through any agent, offered any Common Stock, warrants, Series F Preferred Stock or other securities convertible into Common Stock, Series F Preferred Stock or any security or securities similar to any thereof, for sale to, or

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solicited any offers to buy any Common Stock, warrants, Series F Preferred Stock or other securities convertible into Common Stock, Series F Preferred Stock or any such similar security or securities from, or otherwise approached or negotiated in respect thereof with, any person or entity other than the Investors and a limited number of institutional or sophisticated investors.

4.26 No Governmental Consent or Approval Required. No authorization, consent, approval or other order of, declaration to, or filing with, any governmental agency or body is required to be made or obtained by the Corporation for or in connection with the valid and lawful authorization, execution and delivery by the Corporation of the Transaction Documents, for or in connection with the valid and lawful authorization, issuance, sale and delivery of the Series F Shares or for or in connection with the valid and lawful authorization, reservation, issuance, sale and delivery of the Reserved Shares, except the exemptive filings under applicable securities laws set forth on Schedule 4.26, which are not required to be made until after the Closing and which shall be made on a timely basis.

4.27 Registration Rights. Except as contemplated by the Amended and Restated Registration Rights Agreement, no person has any right to cause the Corporation to effect the registration under the Securities Act of any shares of Common Stock or any other securities of the Corporation.

4.28 Employees. Except as set forth on Schedule 4.28, each employee or consultant now employed or engaged by the Corporation who has access to confidential or proprietary information of the Corporation has executed an agreement with the Corporation relating to, among other matters, non-disclosure of confidential and proprietary information and the assignment of intellectual property, each of which agreements is in full force and effect. The Corporation has complied in all material respects with all applicable state and federal equal opportunity, minimum wage, immigration, workforce reduction and other laws related to employment and termination of employment, except in those cases where such non-compliance would not reasonably be expected to cause a Material Adverse Change. No officer or key employee of the Corporation has advised the Corporation (orally or in writing) that he or she intends to terminate employment with the Corporation.

4.29 Exemptions from Securities Laws. Subject to the accuracy of the representations and warranties of the Investors set forth in Section 5 hereof, the provisions of Section 5 of the Securities Act are inapplicable to the offering, issuance, sale and delivery of the Series F Shares and the Reserved Shares, and no consent, approval, qualification or registration or filing under any state securities laws is required in connection therewith, except the exemptive filings set forth on Schedule 4.26, which are not required to be made until after the Closing and which shall be made on a timely basis.

4.30 Investment Company Act. The Corporation is not an “investment company,” or a company “controlled” by an “investment company,” within the meaning of the Investment Company Act of 1940, as amended.

4.31 Disclosure. Neither this Agreement nor any other certificate, document or written statement furnished to the Investors by or on behalf of the Corporation contains any untrue statement of a material fact or omits to state a material fact necessary in order to make the statements contained herein or therein not misleading.

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SECTION 5. Representations and Warranties of the Investors. Each of the Investors, severally and not jointly, represents and warrants to the Corporation as follows:

5.1 Purchase for Investment. Such Investor is acquiring the Series F Shares purchasable by it hereunder for its own account, for investment and not for, with a view to, or in connection with, any distribution or public offering thereof within the meaning of the Securities Act.

5.2 Unregistered Securities; Legend. Such Investor understands that the Series F Shares and the Reserved Shares have not been, and will not be, registered under the Securities Act or any state securities law, by reason of their issuance in a transaction exempt from the registration requirements of the Securities Act and such laws, that the Series F Shares and the Reserved Shares must be held indefinitely unless they are subsequently registered under the Securities Act and such laws or a subsequent disposition thereof is exempt from registration, that the certificates for the Series F Shares and the Reserved Shares shall bear a legend to such effect, and that appropriate stop transfer instructions may be issued. Such Investor further understands that such exemption depends upon, among other things, the bona fide nature of such Investor’s investment intent expressed herein.

5.3 Status of the Investors. Such Investor has not been formed for the specific purpose of acquiring the Series F Shares pursuant to this Agreement. Such Investor understands the term “accredited investor” as used in Regulation D promulgated under the Securities Act and represents and warrants to the Corporation that such Investor is an “accredited investor” for purposes of acquiring the Series F Shares purchasable by it hereunder.

5.4 Knowledge and Experience; Economic Risk. Such Investor has sufficient knowledge and experience in business and financial matters and with respect to investment in securities of privately held companies so as to enable it to analyze and evaluate the merits and risks of the investment contemplated hereby and is capable of protecting its interest in connection with this transaction. Such Investor is able to bear the economic risk of such investment, including a complete loss of the investment.

5.5 Access to Information. Such Investor acknowledges that such Investor and its representatives have had the opportunity to ask questions and receive answers from officers and representatives of the Corporation concerning the Corporation and its business and the transactions contemplated by this Agreement and to obtain any additional information which the Corporation possesses or can acquire that is necessary to verify the accuracy of the information regarding the Corporation herein set forth or otherwise desired in connection with its purchase of the Series F Shares purchasable by it hereunder.

5.6 Rule 144. Such Investor understands that the exemption from registration afforded by Rule 144 (the provisions of which are known to such Investor) promulgated by the Securities and Exchange Commission under the Securities Act depends upon the satisfaction of various conditions, and that such exemption is not currently available.

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5.7 Place of Business. Such Investor has listed its principal place of business or registered address next to its name on the signature page hereto.

5.8 Authorization of Transaction Documents. Such Investor has duly authorized, executed and delivered the Transaction Documents to which such Investor is a party, and such Transaction Documents constitute the valid and binding obligation of such Investor, enforceable against such Investor in accordance with their terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)).

SECTION 6. Conditions Precedent to Closing by the Investors.

6.1 Closing. The obligation of each Investor to purchase and pay for the Series F Shares being purchased by such Investor at the Closing is subject to satisfaction (or waiver by such Investor) of the following conditions precedent at or before the Closing:

(a) Corporate Proceedings. All corporate and other proceedings to be taken and all waivers and consents to be obtained in connection with the transactions contemplated by this Agreement shall have been taken or obtained and all documents incident to such transactions shall be reasonably satisfactory in form and substance to the Investors and their counsel, who shall have received all such originals or certified or other copies of such documents as they may reasonably request.

(b) Representations and Warranties Correct. The representations and warranties made by the Corporation in Section 4 hereof (each as modified by any schedule referred to therein) shall be true and correct when made, and shall be true and correct in all material respects at the time of the Closing with the same force and effect as if they had been made at and as of the time of the Closing.

(c) Compliance with Covenants. The Corporation shall have duly complied with and performed all covenants and agreements of the Corporation herein which are required to be complied with and performed at or before the Closing.

(d) Certificate of Compliance. The Corporation shall have provided to the Investors a certificate, dated the date of the Closing, in form and substance reasonably satisfactory to the Investors, confirming compliance with the conditions set forth in Sections 6.1(b) and 6.1(c).

(e) Secretary’s Certificate. The Secretary or an Assistant Secretary of the Corporation shall have delivered to the Investors a certificate, dated the date of the Closing, certifying: (a) that attached thereto is a true and complete copy of the By-Laws of the Corporation as in effect on the date of such certification; (b) that attached thereto is a true and complete copy of all resolutions adopted by the Board of Directors and the stockholders of the Corporation authorizing the transactions contemplated by this Agreement and any other actions necessary thereto; (c) that attached thereto is a true and complete copy of the Certificate of Incorporation as in effect on the date of such certification; and (d) to the incumbency and signatures of the officers of the Corporation executing the Transaction Documents.

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(f) Opinion of Counsel. Each of the Investors purchasing Series F Shares shall have received an opinion of Ropes & Gray LLP, counsel for the Corporation, addressed to such Investors, in substantially the form attached hereto as Exhibit 6.1(f).

(g) Related Agreements and Documents. At or before the Closing, the parties thereto shall have executed and delivered the Transaction Documents, and the Corporation shall have delivered to the Investors such other documents consistent with the terms hereof as they shall reasonably request.

(h) Securities Matters. All consents, approvals, qualifications, registrations, notices and filings required to be obtained or effected as of the Closing under any applicable securities laws of any state or other jurisdiction in connection with the issuance, sale and delivery of the Series F Shares and the Reserved Shares shall have been obtained or effected and copies of the same delivered to each of the Investors.

(i) Management Rights Letters. The Corporation shall have executed and delivered to each Investor that so requests a management letter substantially in the form attached hereto as Exhibit 6.1(i).

(j) Delivery of Certificates for Series F Shares. The Corporation shall have delivered to each Investor a certificate for the number and type of Series F Shares being purchased by such Investor at the Closing, each registered in the name of such Investor.

SECTION 7. Conditions Precedent to Closing by the Corporation. The obligation of the Corporation to issue and sell the Series F Shares being sold to the Investors at the Closing is subject to satisfaction (or the waiver by the Corporation) of the following conditions precedent at or before the Closing:

7.1 Representations and Warranties. The representations and warranties made in Section 5 hereof by the Investors purchasing shares at the Closing shall be true and correct when made, and shall be true and correct in all material respects at the time of the Closing with the same force and effect as if they had been made at and as of the time of the Closing.

7.2 Related Agreements and Documents. At or before the Closing, the parties thereto shall have executed and delivered the Transaction Documents.

7.3 Tender of Payment. At the Closing, the Investors shall have tendered payment for the Series F Shares being sold at the Closing.

SECTION 8. Fees; Brokers.

8.1 Fees and Expenses. Each party to this Agreement shall bear all of its own fees and expenses incurred in connection with the preparation and negotiation of this Agreement and the consummation of the transactions contemplated hereby, including all fees of such party’s legal counsel.

13

8.2 Brokers. The Corporation represents and warrants to the Investors that (a) neither the Corporation nor any of its officers, directors, employees or stockholders, has employed any broker or finder in connection with the transactions contemplated by this Agreement, and (b) no person or entity will have, as a result of the transactions contemplated by this Agreement, any right to, interest in, or claim against or upon the Corporation or any Investor for, any commission, fee or other compensation as a finder or broker because of any act or omission by the Corporation or any agent of the Corporation.

SECTION 9. Remedies. In case any one or more of the representations, warranties, covenants or agreements set forth in this Agreement shall have been breached by the Corporation, the Investors may proceed to protect and enforce their rights either by suit in equity or by action at law, including, but not limited to, an action for damages as a result of any such breach or an action for specific performance of any such covenant or agreement contained in this Agreement.

SECTION 10. Indemnification; Limitations on Liability.

10.1 The Corporation shall indemnify, defend and hold the Investors harmless from and against all liabilities, losses, and damages, together with all reasonable costs and expenses related thereto (including, without limitation, reasonable legal and accounting fees and expenses), which would not have been incurred if (a) all of the representations and warranties of the Corporation in the Transaction Documents (each as modified by any schedule referred to therein) had been true and correct when made and at the time of the Closing, (b) all of the covenants and agreements of the Corporation in the Transaction Documents had been duly and timely complied with and performed and (c) a third party claim against such party had not been brought due to the Corporation not duly and timely complying with the advanced notice provisions of the warrants and agreements listed on Schedule 4.11; provided, however, that the aggregate liability of the Corporation to each Investor under this Section 10.1 shall not exceed the aggregate purchase price of the Series F Shares purchased by such Investor hereunder.

SECTION 11. Exchanges; Lost, Stolen or Mutilated Certificates. Upon surrender by any Investor to the Corporation of or any certificate representing Series F Shares or Reserved Shares, the Corporation at its expense shall issue in exchange therefor, and deliver to such Investor, new certificates representing such Series F Shares or Reserved Shares, as the case may be, in such amounts or denominations as may be requested by such Investor. Upon receipt of evidence satisfactory to the Corporation of the loss, theft, destruction or mutilation of any certificate representing any Series F Shares or Reserved Shares and in case of any such loss, theft or destruction, upon delivery of an indemnity agreement satisfactory to the Corporation, or in case of any such mutilation, upon surrender and cancellation of such certificate, the Corporation at its expense shall issue and deliver to such Investor a new certificate for such Series F Shares or Reserved Shares, of like tenor, in lieu of such lost, stolen or mutilated certificate.

14

SECTION 12. Survival of Representations, Warranties and Agreements. The covenants, representations and warranties of the parties contained herein shall survive any Closing hereunder. Each of the parties may rely on such covenants, representations and warranties irrespective of any investigation made, or notice or knowledge held by, it or any other person. All statements contained in any certificate or other instrument delivered by any party pursuant to this Agreement or in connection with the transactions contemplated by this Agreement shall constitute representations and warranties by such party under this Agreement, subject to the qualifications set forth herein and therein.

SECTION 13. Successors and Assigns. This Agreement shall be binding upon, and inure to the benefit of, each of the parties hereto and, except as otherwise expressly provided herein, each other person who shall become a registered holder named in a certificate evidencing Series F Shares or Reserved Shares transferred to such holder by any of the Investors or their permitted transferees, and (except as aforesaid) their respective legal representatives, successors and assigns.

SECTION 14. Entire Agreement; Effect on Prior Documents. This Agreement and the other documents referred to herein or delivered pursuant hereto contain the entire agreement among the parties with respect to the financing transactions contemplated hereby and supersede all prior negotiations, commitments, agreements and understandings among them with respect thereto.

SECTION 15. Notices. All notices, requests, consents and other communications hereunder (“Notices”) to any party shall be contained in a written instrument addressed to such party at the address set forth below or such other address as may hereafter be designated in writing by the addressee to the addressor listing all parties and shall be deemed given (a) when delivered in person or duly sent by fax showing confirmation of receipt, (b) three days after being duly sent by first class mail postage prepaid (other than in the case of Notices to or from any non-U.S. resident, which Notices must be sent in the manner specified in clause (a) or (c)), or (c) two days after being duly sent by DHL, Federal Express or other recognized express international courier service:

(a) if to the Corporation, to:

Acceleron Pharma Inc.

128 Sidney Street

Cambridge, MA 02139

with a copy to:

Ropes & Gray LLP

Prudential Tower

800 Boylston Street

Boston, MA 02119-3600

Attn: Marc Rubenstein

Fax: (617) 951-7050

15

(b) if to the Investors, to their respective addresses as set forth on the Schedule of Investors.

SECTION 16. Amendments; Waivers. This Agreement may be amended, and compliance with the provisions of this Agreement may be omitted or waived, only by the written agreement of the Corporation and Investors or assignees of their rights hereunder holding at least seventy-five percent (75%) in voting power of the then outstanding Series F Shares and Reserved Shares issued upon conversion thereof, provided, however, that no Investor shall, without its consent, be adversely affected by any such modification, amendment or waiver in any manner in which the other Investors are not likewise adversely affected.

SECTION 17. Counterparts. This Agreement may be executed in any number of counterparts, each such counterpart shall be deemed to be an original instrument, and all such counterparts together shall constitute but one agreement. Any such counterpart may contain one or more signature pages.

SECTION 18. Headings. The headings of the various sections of this Agreement have been inserted for convenience of reference only and shall not be deemed to be a part of this Agreement.

SECTION 19. Nouns and Pronouns. Whenever the context may require, any pronouns used herein shall include the corresponding masculine, feminine or neuter forms, and the singular form of names and pronouns shall include the plural and vice-versa.

SECTION 20. Governing Law. This Agreement shall be governed by, and construed and enforced in accordance with, the substantive laws of the Commonwealth of Massachusetts, without regard to its principles of conflicts of laws.

SECTION 21. Severability. Any provision of this Agreement that is prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction, be ineffective to the extent of such prohibition or unenforceability without invalidating the remaining provisions hereof, and any such prohibition or unenforceability in any jurisdiction shall not invalidate or render unenforceable such provision in any other jurisdiction.

[Remainder of page intentionally left blank]

16

IN WITNESS WHEREOF, the undersigned have executed this Series F Convertible Preferred Stock Purchase Agreement as of the day and year first written above.

ACCELERON PHARMA INC.
By:	/s/ John Knopf
	John Knopf, Chief Executive Officer

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

POLARIS VENTURE PARTNERS IV, L.P.
BY: POLARIS VENTURE MANAGEMENT CO. IV, L.L.C.
	ITS GENERAL PARTNER
POLARIS VENTURE PARTNERS ENTREPRENEURS’ FUND IV, L.P.
BY: POLARIS VENTURE MANAGEMENT CO. IV, L.L.C.
	ITS GENERAL PARTNER
By:	/s/ William E. Bilodeau
	William E. Bilodeau
	Attorney-in-fact
Address: 1000 Winter Street, Suite 3350                 Waltham, MA 02451

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

VENROCK PARTNERS, L.P.,

by its General Partner, Venrock Partners Management, LLC

VENROCK ASSOCIATES IV, L.P.,

by its General Partner, Venrock

Management IV, LLC

VENROCK ENTREPRENEURS FUND IV, L.P.,

by its General Partner, VEF Management IV, LLC

Address: 530 Fifth Avenue, 22nd Floor

                New York, NY 10036

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

ADVANCED TECHNOLOGY VENTURES VII, L.P.		ADVANCED TECHNOLOGY VENTURES VII(C), L.P.
By:	ATV Associates VII, L.L.C., Its General Partner	By:	ATV Associates VII, L.L.C. its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
ADVANCED TECHNOLOGY VENTURES VI, L.P.		ATV ALLIANCE 2003, L.P.
By:	ATV Associates VI, L.L.C., its General Partner	By:	ATV Alliance Associates, L.L.C., Its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
ADVANCED TECHNOLOGY VENTURES VII (B), L.P.		ATV ENTREPRENEURS VI, L.P.
By:	ATV Associates VII, L.L.C., its General Partner	By:	ATV Associates VI, L. L. C., its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
ATV ENTREPRENEURS VII, L.P.
By:	ATV Associates VII, L.L.C., its General Partner
By:	/s/ Jean George
Name:	Jean George
Title:	Managing Director
		Address: Bay Colony Corporate Center                 1000 Winter Street, Suite 3700                 Waltham, MA 02451

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22 day of December, 2011.

	CELGENE CORPORATION
	By:	/s/ Perry Karsen
	Name:	Perry Karsen
	Title:	Chief Operations Officer
	Address:	86 Morris Avenue Summit, NJ 07901

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

ORBIMED PRIVATE INVESTMENTS II, LP		ORBIMED PRIVATE INVESTMENTS II (QP), LP
By:	Orbimed Capital GP II LLC its General Partner	By:	Orbimed Capital GP II LLC its General Partner
By:	/s/ Carl Gordon	By:	/s/ Carl Gordon
Name:	Carl Gordon	Name:	Carl Gordon
Title:	Member	Title:	Member
UBS JUNIPER CROSSOVER FUND, L.L.C.
By:	Orbimed Advisors LLC its Member
By:	/s/ Carl Gordon
Name:	Carl Gordon
Title:	Member

Address:	767 Third Avenue, 30th Floor
	New York, NY 10017

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

APPLIED GENOMIC TECHNOLOGY

CAPITAL FUND, L.P.; AGTC ADVISORS

FUND, L.P.

Each by: AGTC Partners, L.P., its General

Partner

By: NewcoGen Group Inc., its General

Partner

Address: 1 Memorial Drive, 7th Fl.

                Cambridge, MA 02142

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

BESSEMER VENTURE PARTNERS VII

L.P.,

BESSEMER VENTURE PARTNERS VII

INSTITUTIONAL L.P.

By: Deer VII & Co. L.P., their General

Partner

By: Deer VII & Co. Ltd., its General Partner

By:	/s/ Edmund Colloton
Name:	Edmund Colloton
Title:	Director

Address: c/o Bessemer Venture Partners

                1865 Palmer Avenue, Suite 104

                Larchmont, NY 10538

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

ALKERMES, INC.
By:	/s/ Michael Landine
Name:	MICHAEL LANDINE
Title:	SENIOR VICE PRESIDENT
Address:	852 Winter Street Waltham, MA 02451

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

SUTTER HILL VENTURES, A CALIFORNIA LIMITED PARTNERSHIP		DAVID L. ANDERSON, TRUSTEE OF THE ANDERSON LIVING TRUST U/A/D 1/22/98
By:	Sutter Hill Ventures, L.L.C.
	its General Partner
By:	/s/ Jeffrey W. Bird	By:	/s/ Robert Yin
Name:	Jeffrey W. Bird		David L. Anderson, Trustee
Title:	Managing Director		By Robert Yin Under Power of Attorney
ANVEST, L.P.		G. LEONARD BAKER, JR. AND MARY ANNE BAKER, CO-TRUSTEES OF THE BAKER REVOCABLE TRUST U/A/D 2/3/03
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	David L. Anderson, Trustee of The Anderson Living		G. Leonard Baker, Jr., Trustee
	Trust U/A/D 1/22/98, General Partner
	By Robert Yin		By Robert Yin
	Under Power of Attorney		Under Power of Attorney
SAUNDERS HOLDINGS, L.P.		YOVEST, L.P.
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	G. Leonard Baker, Jr., Trustee of The Baker Revocable Trust U/A/D 2/3/03, General Partner		William H. Younger, Jr., Trustee of The William H. Younger, Jr. Revocable Trust U/A/D 8/5/09, General Partner
	By Robert Yin		By Robert Yin
	Under Power of Attorney		Under Power of Attorney
ROOSTER PARTNERS, LP		GREGORY P. SANDS AND SARAH J.D. SANDS AS TRUSTEES OF GREGORY P. AND SARAH J.D. SANDS TRUST AGREEMENT DATED 2/24/99
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	Tench Coxe, Trustee of The Coxe Revocable Trust U/A/D 4/23/98, General Partner		Gregory P. Sands, Trustee
	By Robert Yin		By Robert Yin
	Under Power of Attorney		Under Power of Attorney
JAMES C. GAITHER, TRUSTEE OF THE GAITHER REVOCABLE TRUST U/A/D 9/28/2000		TALLACK PARTNERS, L.P.
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	James C. Gaither Trustee		James C. Gaither, Trustee of The Gaither Revocable Trust U/A/D 9/28/2000, General Partner
	By Robert Yin		By Robert Yin
	Under Power of Attorney		Under Power of Attorney

Signature Page to Stock Purchase Agreement

JAMES N. WHITE AND PATRICIA A. O’BRIEN AS TRUSTEES OF THE WHITE FAMILY TRUST U/A/D 4/3/97		JEFFREY W. BIRD AND CHRISTINA R. BIRD AS TRUSTEES OF JEFFREY W. AND CHRISTINA R. BIRD TRUST AGREEMENT DATED 10/31/00
By:	/s/ Robert Yin	By:	/s/ Jeffrey W. Bird
	James N. White Trustee		Jeffrey W. Bird, Trustee
	By Robert Yin
	Under Power of Attorney
ANDREW T. SHEEHAN AND NICOLE J. SHEEHAN AS TRUSTEES OF SHEEHAN 2003 TRUST		MICHAEL L. SPEISER AND MARY ELIZABETH SPEISER, CO TRUSTEES OF SPEISER TRUST AGREEMENT DATED 7/19/06
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	Andrew T. Sheehan, Trustee		Michael L. Speiser, Trustee
	By Robert Yin		By Robert Yin
	Under Power of Attorney		Under Power of Attorney

Address: 755 Page Mill Road, Suite A-200 Palo Alto, CA 94304-1005

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Sherryl W. Casella	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Robert Yin
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO David E. Sweet	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Lynne B. Graw
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Diane J. Narr	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Yu-Ying Chen
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Patricia Tom (Post)	Address: Wells Fargo Bank, N.A. Attention: Tom Thurston 600 California Street 12th Floor MAC A0193-120 San Francisco, CA 94108 Phone (415) 396-3736 Fax (415) 975-7539 Email: thursttm@wellsfargo.com The above WFB address is for overnite mail (Fed Ex, etc.) For mailing, the address is Ms. Tom Thurston Wells Fargo Bank MAC-A0101-021 P.O. Box 63050 San Francisco, CA 94163

By:	/s/ Thomas M. Thurston
Name:	Thomas M. Thurston
Title:	Vice President

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

QVT FUND LP,		QUINTESSENCE FUND L.P.,
BY: ITS GENERAL PARTNER, QVT ASSOCIATES GP LLC		BY: ITS GENERAL PARTNER, QVT ASSOCIATES GP LLC
By:	/s/ Keith S. Manchester	By:	/s/ Keith S. Manchester
Name:	Keith S. Manchester	Name:	Keith S. Manchester
Title:	Portfolio Manager	Title:	Portfolio Manager

Address:	c/o QVT Financial LP
	1177 Avenue of the Americas
	9th Floor
	New York, NY 10036

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

AVALON VENTURES VI, LP		AVALON VENTURES VI, GP FUND, LLC
By:	/s/ Douglas Downs	By:	/s/ Douglas Downs
Name:	Douglas Downs	Name:	Douglas Downs
Title:	Authorized Signer & CFO	Title:	Authorized Signer & CFO

Address:	1134 Kline St
	La Jolla, CA 92037

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

MIDCAP FINANCIAL, LLC, a Delaware limited liability company
By:	/s/ Luis Viera
Name:	Luis Viera
Title:	Managing Director
Address:	7255 Woodmont Avenue Suite 200 Bethesda, MD 20814 Attention: Mr. Bob Goodridge

Signature Page to Stock Purchase Agreement

ACCELERON PHARMA INC.

Series F Convertible Preferred Stock

Purchase Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned investor hereby agrees (i) that it is an “Investor” as defined in the Series F Convertible Preferred Stock Purchase Agreement dated as of December 22, 2011 among Acceleron Pharma Inc. and the other parties thereto (the “Purchase Agreement”), (ii) that it is a party to the Purchase Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Purchase Agreement.

EXECUTED this 22^nd day of December, 2011.

HERCULES TECHNOLOGY II, L.P.
By:	/s/ K. Nicholas Martitsch
Name:	K. Nicholas Martitsch
Title:	Associate General Counse|
Address:	400 Hamilton Avenue Suite 310 Palo Alto, CA 94301

Signature Page to Stock Purchase Agreement

EX-99.6

Exhibit 6

REGISTRATION RIGHTS AGREEMENT

(AMENDED AND RESTATED AS OF DECEMBER 22, 2011)

This Amended and Restated Registration Rights Agreement (this “Agreement”) is made as of December 22, 2011 by and among Acceleron Pharma Inc., a Delaware corporation (the “Corporation”) and the parties listed on Exhibit A hereto (the “Investors”).

WHEREAS, the Corporation and certain of the Investors (the “Existing Investors”) are parties to that certain Amended and Restated Registration Rights Agreement dated as of June 10, 2010 (the “Prior Agreement”).

WHEREAS, the Existing Investors executing signature pages hereto hold at least two-thirds in voting power of the outstanding shares of Registerable Shares (as such capitalized term is defined in the Prior Agreement) and therefore may validly join with the Corporation to amend the terms of the Prior Agreement pursuant to Section 19 thereof.

WHEREAS, in connection with the sale by the Corporation of Series F Convertible Preferred Stock to certain of the Investors as of the date hereof, the Existing Investors and the Corporation desire to amend and restate the Prior Agreement as provided herein.

NOW, THEREFORE, in consideration of the premises and the mutual agreements hereinafter set forth, the parties hereto agree as follows:

SECTION 1. Definitions. As used in this Agreement, the following terms shall have the following meanings:

(a) The term “1934 Act” means the Securities Exchange Act of 1934, as amended.

(b) The term “Common Stock” means the Corporation’s Common Stock, $.001 par value per share.

(c) The term “Holder” means any holder of Registrable Shares.

(d) The terms “register,” “registered,” and “registration” refer to a registration effected by preparing and filing a registration statement in compliance with the Securities Act and the declaration or ordering of effectiveness of such registration statement.

(e) The term “Preferred Stock” means the Corporation’s Series A Convertible Preferred Stock, par value $.001 per share, the Corporation’s Series B Convertible Preferred Stock, par value $.001 per share, the Corporation’s Series C Convertible Preferred Stock, par value $.001 per share, the Corporation’s Series C-1 Convertible Preferred Stock, par value $.001 per share, the Corporation’s Series D Convertible Preferred Stock, par value $.001 per share, the Corporation’s Series D-1 Convertible Preferred Stock, par value $.001 per share, the Corporation’s Series E Convertible Preferred Stock, par value $.001 per share, and the Corporation’s Series F Convertible Preferred Stock, par value $.001 per share.

(f) The term “Registrable Shares” means (1) the Common Stock issuable upon conversion of Preferred Stock, (2) any Common Stock purchased by an Investor (or its permitted transferees) pursuant to Section 3 of the Amended and Restated Investor Rights Agreement of even date herewith by and among the Corporation and the Investors (or Common Stock issuable with respect to other securities so purchased), (3) for purposes of registrations described in Section 3 of this Agreement only, the Common Stock issuable upon the exercise of warrants issued by the Corporation to RRD International LLC or any affiliate (as defined in Rule 144) of RRD International LLC, (4) for purposes of registrations described in Section 3 of this Agreement only, the Common Stock issuable upon the exercise of warrants issued by the Corporation to General Electric Capital Corporation or any affiliate (as defined in Rule 144) of General Electric Capital Corporation, (5) the Common Stock issuable upon the conversion of the Preferred Stock, or other securities, issuable upon the exercise of warrants issued by the Corporation to Hercules Technology II, L.P. pursuant to the Warrant Agreement dated as of December 21, 2005, (6) the Common Stock issuable upon the exercise of warrants issued by the Corporation to Midcap Funding I, LLC, Silicon Valley Bank and Oxford Finance Corporation or any affiliate (as defined in Rule 144) thereof (collectively, the “Oxford Warrants”), (7) the Common Stock issuable upon the exercise or conversion of Warrants issued by the Corporation to certain of the Investors pursuant to the Series E Stock and Warrant Purchase Agreement dated as of June 10, 2010, and (8) any Common Stock of the Corporation issued as a dividend or other distribution with respect to, or in exchange or in replacement of, such Preferred Stock or Common Stock.

(g) The term “Rule 144” means Rule 144 promulgated under the Securities Act.

(h) The term “SEC” means the Securities and Exchange Commission.

(i) The term “Securities Act” means the Securities Act of 1933, as amended.

In addition, for purposes of all calculations and notices under this Agreement, and all other provisions of this Agreement where the context permits, a holder of Preferred Stock shall be deemed the Holder of the Registrable Shares issuable upon conversion thereof, and such Preferred Stock shall be deemed outstanding Registrable Shares hereunder. Notwithstanding the foregoing, nothing in this Agreement shall require the Corporation actually to register any shares of Preferred Stock.

SECTION 2. Request for Registration. If at any time after the earlier to occur of (i) the fourth anniversary of the date of this Agreement and (ii) the date six months after the first public offering of the Corporation’s securities, the Corporation shall receive a written request (specifying that it is being made pursuant to this Section 2) from a majority of the holders of Preferred Stock (other than the holders of the Oxford Warrants) that the Corporation file a registration statement under the Securities Act, or a similar document pursuant to any other statute then in effect corresponding to the Securities Act, covering the registration of Registrable Shares the expected price to the public of which equals or exceeds $5,000,000 (based on the market price or fair value on the date of such request), then the Corporation shall promptly notify all other Holders (including the holders of the Oxford Warrants) of such request and shall use its

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best efforts to cause all Registrable Shares that Holders have requested to be registered under the Securities Act on Form S-1 or any other available form the use of which is approved by the Holders of a majority of the Registrable Shares that are to be included in such registration.

Notwithstanding the foregoing, (i) the Corporation shall not be obligated to effect a registration pursuant to this Section 2 during the period starting with the date sixty (60) days prior to the Corporation’s estimated date of filing of, and ending on a date six (6) months following the effective date of, a registration statement pertaining to an underwritten public offering of securities for the account of the Corporation; provided, that the Corporation is actively employing in good faith its best efforts to cause such registration statement to become effective and that the Corporation’s estimate of the date of filing such registration statement is made in good faith; (ii) the Corporation shall not be obligated to effect a registration pursuant to this Section 2 within six (6) months after the effective date of a prior registration under this Section 2; and (iii) if the Corporation shall furnish to the Holders a certificate signed by the President of the Corporation stating that in the good faith judgment of the Board of Directors it would be seriously detrimental to the Corporation or its shareholders for a registration statement to be filed in the near future, then the Corporation’s obligation to use its best efforts to file a registration statement shall be deferred for a period not to exceed 90 days; provided, however, that the Corporation shall not be permitted to so defer its obligation more than once in any 12-month period.

The Corporation shall not be obligated to effect more than two registrations on behalf of the Holders pursuant to this Section 2.

SECTION 3. Corporation Registration. If at any time the Corporation proposes to register any of its Common Stock under the Securities Act in connection with the public offering of such securities for its own account or for the accounts of shareholders other than Holders, solely for cash on a form that would also permit the registration of the Registrable Shares, the Corporation shall, each such time, promptly give each Holder written notice of such determination. Upon the written request of any Holder given within thirty (30) days after giving of any such notice by the Corporation, the Corporation shall, subject to the limitations set forth in Section 8, use its best efforts to cause to be registered under the Securities Act all of the Registrable Shares that each such Holder has requested be registered; provided, that the Corporation shall have the right to postpone or withdraw any registration statement relating to an offering in which the Holders are eligible to participate under this Section 3 without any liability or obligation to the Holders under this Section 3. Further, if all the Investors eligible under this Section 3 to require that their Registrable Shares be included in the registration covering the Corporation’s initial public offering waive their right under this Section 3 with respect to the Corporation’s initial public offering, then none of RRD International LLC, General Electric Capital Corporation and Hercules Technology II, L.P. shall have any rights under this Sections 3 to require Registerable Shares held by them or their affiliates (as defined in Rule 144) to be included in the Corporation’s initial public offering.

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SECTION 4. Obligations of the Corporation. Whenever required under Section 2, Section 3 or Section 11 to use its best efforts to effect the registration of any Registrable Shares, the Corporation shall, as expeditiously as reasonably possible:

(a) Prepare and file with the SEC a registration statement with respect to such Registrable Shares and use its best efforts to cause such registration statement to become effective, and, upon the request of the Holders of a majority of the Registrable Shares registered thereunder, keep such registration statement effective for a period of up to one hundred twenty (120) days or, if earlier, until the distribution contemplated in the registration statement has been completed; provided, however, that (i) such 120-day period shall be extended for a period of time equal to the period the Holder refrains from selling any securities included in such registration at the request of an underwriter of Common Stock (or other securities) of the Corporation; and (ii) in the case of any registration of Registrable Shares on Form S-3 which are intended to be offered on a continuous or delayed basis, subject to compliance with applicable SEC rules, such 120-day period shall be extended for up to 90 days, if necessary, to keep the registration statement effective until all such Registrable Shares are sold.

(b) Prepare and file with the SEC such amendments and supplements to such registration statement and the prospectus used in connection with such registration statement as may be necessary to comply with the provisions of the Securities Act with respect to the disposition of all securities covered by such registration statement.

(c) Furnish to the selling Holders such numbers of copies of a prospectus, including a preliminary prospectus, in conformity with the requirements of the Securities Act, and such other documents as they may reasonably request in order to facilitate the disposition of such Registrable Shares owned by them.

(d) Use its best efforts to register and qualify the securities covered by such registration statement under such other securities or Blue Sky laws of such jurisdictions as shall be reasonably appropriate for the distribution of the securities covered by the registration statement; provided, that the Corporation shall not be required in connection therewith or as a condition thereto to qualify to do business or to file a general consent to service of process in any such states or jurisdictions, and further provided that (anything in this Agreement to the contrary notwithstanding with respect to the bearing of expenses) if any jurisdiction in which the securities shall be qualified shall require that expenses incurred in connection with the qualification of the securities in that jurisdiction be borne by selling shareholders, then such expenses shall be payable by selling shareholders pro rata, to the extent required by such jurisdiction.

(e) Promptly notify each selling Holder and each underwriter and (if requested by any such person) confirm such notice in writing (A) when a prospectus or any prospectus supplement or post-effective amendment has been filed and, with respect to a registration statement or any post-effective amendment, when the same has become effective, (B) of the issuance by any state securities or other regulatory authority of any order suspending the qualification or exemption from qualification of any of the Registrable Shares under state securities or Blue Sky laws or the initiation, or threatened initiation, of any proceedings for that purpose, or (C) of the happening of any event which requires the making of any changes in a registration statement or prospectus so that they shall not contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading, and, as promptly as practicable thereafter, prepare and file

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with the SEC and furnish a supplement or amendment to such prospectus so that, as thereafter deliverable to the purchasers of such Registrable Shares, such prospectus shall not contain any untrue statement of a material fact or omit a material fact necessary to make the statements therein, in light of the circumstances under which they were made, not misleading.

(f) Notify each selling Holder of Registrable Shares that might reasonably be deemed to be an underwriter or controlling person of the Corporation promptly of any request by the SEC for the amending or supplementing of such registration statement or prospectus or for additional information.

(g) Provide a transfer agent for the Common Stock no later than the effective date of the first registration of any Registrable Shares.

(h) Otherwise use its best efforts to comply with all applicable rules and regulations of the SEC.

(i) Use its best efforts either (i) to cause all such Registrable Shares to be listed on a national securities exchange (if such securities are not already so listed) and on each additional national securities exchange on which similar securities issued by the Corporation are then listed, if the listing of such securities is then permitted under the rules of such exchange, or (ii) to secure designation of all such Registrable Shares as a Nasdaq “national market system security” within the meaning of Rule 11Aa2-1 of the SEC or, failing that, to secure listing on Nasdaq for such Registrable Shares and, without limiting the generality of the foregoing, to arrange for at least two (2) market makers to register as such with respect to Registrable Shares with the National Association of Securities Dealers.

(j) Enter into such customary agreements (including an underwriting agreement in customary form) and take such other actions as the selling Holders of Registrable Shares shall reasonably request in order to expedite or facilitate the disposition of such Registrable Shares.

(k) Make available for inspection by any selling Holder of Registrable Shares, by any underwriter participating in any disposition to be effected pursuant to such registration statement and by any attorney, accountant or other agent retained by any such selling Holder or any such underwriter, all pertinent financial and other records and pertinent corporate documents and properties of the Corporation, and cause all of the Corporation’s officers, directors and employees to supply all information reasonably requested by any such selling Holder, underwriter, attorney, accountant or agent in connection with such registration statement.

(l) Use every reasonable effort to prevent the issuance of any stop order suspending the effectiveness of such registration statement or of any order preventing or suspending the use of any preliminary prospectus and, if any such order is issued, to obtain the lifting thereof at the earliest reasonable time.

(m) Make such representations and warranties to the selling Holders of Registrable Shares and the underwriters as are customarily made by issuers to selling stockholders and underwriters, as the case may be, in primary underwritten public offerings.

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SECTION 5. Furnish Information. It shall be a condition precedent to the obligations of the Corporation to take any action pursuant to this Agreement with respect to the registration of any Holder’s Registrable Shares that such Holder shall take such actions and furnish to the Corporation such information regarding itself, the Registrable Shares held by it, and the intended method of disposition of such securities, as the Corporation shall reasonably request and as shall be required in connection with any registration, qualification or compliance referred to in this agreement, including, without limitation (i) in connection with an underwritten offering, enter into an appropriate underwriting agreement containing terms and provisions then customary in agreements of that nature, (ii) enter into such custody agreements, powers of attorney and related documents at such time and on such terms and conditions as may then be customarily required in connection with such offering and (iii) distribute the Registrable Shares only in accordance with and in the manner of the distribution contemplated by the applicable registration statement and prospectus. In addition, the Holders shall promptly notify the Corporation of any request by the Commission or any state securities commission or agency for additional information or for such registration statement or prospectus to be amended or supplemented.

SECTION 6. Expenses of Demand Registration. All expenses incurred in connection with any registration pursuant to Section 2 or Section 11 (excluding underwriters’ discounts and commissions), including, without limitation, all registration and qualification fees, printers’ and accounting fees, fees and disbursements of counsel for the Corporation, and the reasonable fees and disbursements of one special counsel for the selling Holders collectively, shall be borne by the Corporation; provided, however, that the Corporation shall not be required to pay for any expenses of any registration proceeding begun pursuant to Section 2 if the registration request is subsequently withdrawn at the request of the Holders of a majority of the Registrable Shares to be registered (in which case all participating Holders shall bear such expenses pro rata based upon the number of Registrable Shares that were to be included in the withdrawn registration), unless the Holders of a majority of the Registrable Shares agree to forfeit their right to one demand registration pursuant to Section 2; provided further, however, that if at the time of such withdrawal, the Holders have learned of a material adverse change in the condition, business, or prospects of the Corporation from that known to the Holders at the time of their request and have withdrawn the request with reasonable promptness after learning of such information, then the Holders shall not be required to pay any of such expenses and shall retain their rights pursuant to Section 2.

SECTION 7. Corporation Registration Expenses. All expenses (excluding underwriters’ discounts and commissions) incurred in connection with any registration pursuant to Section 3, including, without limitation, any additional registration and qualification fees and any additional fees and disbursements of counsel to the Corporation that result from the inclusion of securities held by the selling Holders in such registration and the reasonable fees and disbursements of one special counsel for the selling Holders collectively, shall be borne by the Corporation.

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SECTION 8. Underwriting Requirements.

(a) In connection with any offering under Section 3 involving an underwriting of shares being issued by the Corporation, the Corporation shall not be required to include any Holder’s Registrable Shares in such underwriting unless such Holder accepts the terms of the underwriting as agreed upon between the Corporation and the underwriters selected by it, and then only in such quantity as will not, in the reasonable opinion of the underwriters, jeopardize the success of the offering by the Corporation. If the total amount of securities that all Holders request to be included in an underwritten offering under Section 3 exceeds the amount of securities that the underwriters reasonably believe compatible with the success of the offering, no securities of any shareholder shall be included in such offering, except (i) securities of the shareholder, if any, on whose behalf the registration is undertaken, (ii) securities included in such underwritten offering pursuant to the exercise of contractual demand registration rights and (iii) Registrable Shares of Holders, unless all Registrable Shares which the Holders have requested to be included are included, and the Corporation shall only be required to include in the offering so many of the Registrable Shares of the Holders as the underwriters reasonably believe will not jeopardize the success of the offering (the Registrable Shares so included to be apportioned pro rata among the selling Holders according to the total amount of Registrable Shares owned by such selling Holders, or in such other proportions as shall mutually be agreed to by such selling Holders).

(b) With respect to any underwriting of shares to be registered under Section 2 or Section 11, the selling Holders who initiate the request for registration shall have the right to designate the managing underwriter or underwriters, subject to the consent of the Corporation. In connection with any underwritings of shares to be registered under Section 3, the Corporation shall have the right to designate the managing underwriter or underwriters. In any such case, such consent of the Corporation or the Holders shall not be unreasonably withheld or delayed.

SECTION 9. Delay of Registration. No Holder shall have any right to take any action to restrain, enjoin, or otherwise delay any registration as the result of any controversy that might arise with respect to the interpretation or implementation of this Agreement.

SECTION 10. Indemnification. In the event any Registrable Shares are included in a registration statement under this Agreement:

(a) To the extent permitted by law, the Corporation will indemnify and hold harmless each Holder requesting or joining in a registration, the partners, members, officers, directors and stockholders of such Holder, legal counsel and accountants for such Holder, any underwriter (as defined in the Securities Act) for such Holder and each person, if any, who controls such Holder or underwriter within the meaning of the Securities Act or the 1934 Act, against any losses, claims, damages or liabilities, joint or several, to which they may become subject under the Securities Act or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based on (i) any untrue or alleged untrue statement of any material fact contained in such registration statement, including, without limitation, any preliminary prospectus or final prospectus contained therein or any amendments or supplements thereto, (ii) the omission or alleged omission to state therein a material fact required to be stated therein, or necessary to make the statements therein not misleading or (iii) any violation by the Corporation of any rule or regulation promulgated under the Securities Act applicable to the Corporation and relating to action or inaction required of the Corporation in connection with any such registration; and will promptly reimburse each such Holder,

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underwriter, controlling person or other aforementioned person for any legal or other expenses reasonably incurred by them in connection with investigating or defending any such loss, claim, damage, liability, or action; provided, however, that the indemnity agreement contained in this Section 10(a) shall not apply to amounts paid in settlement of any such loss, claim, damage, liability or action if such settlement is effected without the consent of the Corporation (which consent shall not be unreasonably withheld or delayed) nor shall the Corporation be liable to any such Holder, underwriter, controlling person or other aforementioned person in any such case for any such loss, claim, damage, liability or action to the extent that it (i) arises out of or is based upon an untrue statement or alleged untrue statement or omission or alleged omission made in connection with such registration statement, preliminary prospectus, final prospectus, or amendments or supplements thereto, in reliance upon and in conformity with written information furnished to the Corporation expressly for use in connection with such registration by or on behalf of such Holder, underwriter, controlling person or other aforementioned person, (ii) is caused by the failure of such Holder to deliver a copy of the final prospectus relating to such Registrable Shares, as then amended or supplemented, in connection with a purchase, if the Corporation had previously furnished copies thereof to such Holder or (iii) is caused by such Holder’s disposition of Registrable Shares during any period during which such Holder is obligated to discontinue any disposition of Registrable Shares under Section 17.

(b) To the extent permitted by law, each Holder requesting or joining in a registration will, severally and not jointly, indemnify and hold harmless the Corporation, each of its directors, each of its officers who has signed the registration statement, each person, if any, who controls the Corporation within the meaning of the Securities Act, and any underwriter (within the meaning of the Securities Act) for the Corporation against any losses, claims, damages or liabilities to which the Corporation or any such director, officer, controlling person or underwriter may become subject, under the Securities Act or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereto) arise out of or are based upon (i) any untrue statement or alleged untrue statement of any material fact contained in such registration statement, including any preliminary prospectus or final prospectus contained therein or any amendments or supplements thereto or (ii) the omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, in each case to the extent, but only to the extent, that such untrue statement or alleged untrue statement or omission or alleged omission was made in reliance upon and in conformity with written information relating to and furnished to the Corporation by such Holder expressly for use in connection with such registration; and will promptly reimburse the Corporation or any such director, officer, controlling person or underwriter for any legal or other expenses reasonably incurred by them in connection with investigating or defending any such loss, claim, damage, liability or action; provided, however, that the indemnity agreement contained in this Section 10(b) shall not apply to amounts paid in settlement of any such loss, claim, damage, liability or action if such settlement is effected without the consent of such Holder (which consent shall not be unreasonably withheld or delayed) and provided further that no Holder shall have any liability under this Section 10(b) in excess of the net proceeds actually received by such Holder in the relevant public offering.

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(c) Promptly after receipt by an indemnified party under this Section 10 of notice of the commencement of any action, such indemnified party will, if a claim in respect thereof is to be made against any indemnifying party under this Section 10, notify the indemnifying party in writing of the commencement thereof and the indemnifying party shall have the right to participate in, and, to the extent the indemnifying party so desires, jointly with any other indemnifying party similarly noticed, to assume the defense thereof with counsel mutually satisfactory to the parties. The failure to notify an indemnifying party promptly of the commencement of any such action, if prejudicial to his ability to defend such action, shall relieve such indemnifying party of any liability to the indemnified party under this Section 10, but the omission so to notify the indemnifying party will not relieve him of any liability that he may have to any indemnified party otherwise than under this Section 10.

(d) If the indemnification provided for in this Section 10 is required by its terms but is for any reason held to be unavailable to or otherwise insufficient to hold harmless an indemnified party under Section 10(a) or Section 10(b) in respect of any losses, claims, damages, liabilities or expenses referred to herein, then each applicable indemnifying party shall contribute to the amount paid or payable by such indemnified party as a result of any losses, claims, damages, liabilities or expenses referred to herein in such proportion as is appropriate to reflect the relative fault of the Corporation and the selling Holders in connection with the statements or omissions described in such Section 10(a) or Section 10(b) which resulted in such losses, claims, damages, liabilities or expenses, as well as any other relevant equitable considerations. The relative fault of the Corporation and the selling Holders shall be determined by reference to, among other things, whether the untrue or alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information supplied by the Corporation or the selling Holders and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission. The amount paid or payable by a party as a result of the losses, claims, damages, liabilities and expenses referred to above shall be deemed to include, subject to the limitations set forth in this Section 10, any legal or other fees or expenses reasonably incurred by such party in connection with investigating or defending any action or claim. The provisions set forth in Section 10(c) with respect to notice of commencement of any action shall apply if a claim for contribution is to be made under this Section 10(d); provided, however, that no additional notice shall be required with respect to any action for which notice has been given under subsection Section 10(c) for purposes of indemnification. The Corporation and the selling Holders agree that it would not be just and equitable if contribution pursuant to this Section 10 were determined solely by pro rata allocation or by any other method of allocation which does not take account of the equitable considerations referred to in this paragraph. Notwithstanding the provisions of this Section 10(d), no Holder shall be required to contribute an amount in excess of the net proceeds actually received by such Holder in the relevant public offering. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Securities Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation.

(e) Notwithstanding the foregoing, to the extent that the provisions on indemnification contained in the underwriting agreements entered into among the Holders, the Corporation and the underwriters in connection with an underwritten public offering are in conflict with the foregoing provisions, the provisions in the underwriting agreement shall be controlling as to the Registrable Shares included in the public offering.

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SECTION 11. Registrations on Form S-3.

(a) If (i) the Corporation shall receive a written request (specifying that it is being made pursuant to this Section 11) from one or more Holders that the Corporation file a registration statement on Form S-3 (or any successor form to Form S-3 regardless of its designation) for a public offering of Registrable Shares the reasonably anticipated aggregate price to the public of which would equal or exceed $1,000,000, and (ii) the Corporation is a registrant entitled to use Form S-3 (or any successor form to Form S-3) to register such shares, then the Corporation shall promptly notify all other Holders of such request and shall use its best efforts to cause all Registrable Shares that Holders have requested be registered to be registered on Form S-3 (or any successor form to Form S-3).

(b) Notwithstanding the foregoing, (i) the Corporation shall not be obligated to effect a registration pursuant to this Section 11 during the period starting with the date sixty (60) days prior to the Corporation’s estimated date of filing of, and ending on a date six (6) months following the effective date of, a registration statement pertaining to an underwritten public offering of securities for the account of the Corporation; provided, that the Corporation is actively employing in good faith its best efforts to cause such registration statement to become effective and that the Corporation’s estimate of the date of filing such registration statement is made in good faith; (ii) the Corporation shall not be obligated to effect a registration pursuant to this Section 11 within six (6) months after the effective date of a prior registration under this Section 11; and (iii) if the Corporation shall furnish to the Holders a certificate signed by the President of the Corporation stating that in the good faith judgment of the Board of Directors it would be seriously detrimental to the Corporation or its shareholders for a registration statement to be filed in the near future, then the Corporation’s obligation to use its best efforts to file a registration statement shall be deferred for a period not to exceed 90 days; provided, however, that the Corporation shall not be permitted to so defer its obligation more than once in any 12-month period.

(c) The Holders’ rights to registration under this Section 11 are in addition to, and not in lieu of, their rights to registration under Section 2 and Section 3 of this Agreement.

SECTION 12. Limitation on Corporation Offerings. The Corporation shall not register securities for sale for its own account (or, except as permitted by Section 14, any securities other than Registrable Shares) in any registration requested pursuant to Section 2 or Section 11 unless permitted to do so by the written consent of the Holders of a majority of the Registrable Shares as to which registration has been requested unless the inclusion of securities for the account of the Corporation would not require a reduction in the number of Registrable Shares to be included in such registration, as determined by the managing underwriter.

SECTION 13. Reports Under Securities Exchange Act of 1934. With a view to making available to the Holders the benefits of Rule 144 and any other rule or regulation of the SEC that may at any time permit a Holder to sell securities of the Corporation to the public without registration, the Corporation agrees to use its best efforts to:

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(a) make and keep public information available, as those terms are understood and defined in Rule 144, at all times subsequent to 90 days after the effective date of the first registration statement covering an underwritten public offering filed by the Corporation;

(b) file with the SEC in a timely manner all reports and other documents, if any, required of the Corporation under the Securities Act and the 1934 Act and take such further action as the Holders may reasonably request, all to the extent required from time to time to enable the Holders to sell securities of the Corporation without registration; and

(c) furnish to any Holder forthwith upon request a written statement by the Corporation that it has complied with the reporting requirements of Rule 144 (at any time after 90 days after the effective date of said first registration statement filed by the Corporation), and of the Securities Act and the 1934 Act (at any time after it has become subject to such reporting requirements), a copy of the most recent annual or quarterly report of the Corporation, and such other reports and documents so filed by the Corporation as may be reasonably requested in availing any such holder to take advantage of any rule or regulation of the SEC permitting the selling of any such securities without registration.

SECTION 14. Limitations in Connection with Future Grants of Registration Rights. Without the prior written consent of the Holders of at least two-thirds in voting power of then outstanding Registrable Shares held by Investors, the Corporation shall not grant rights to any person or entity: (a) to cause the Corporation to register any of such person’s or entity’s securities of the Corporation; (b) to include such person’s or entity’s securities of the Corporation in any registration statement filed under Section 2 or Section 11 hereof; (c) to include such person’s or entity’s securities of the Corporation in any registration statement described in Section 3 hereof, unless, in the case of each of (a), (b) and (c), under the terms of such agreement, such person or entity may include such securities in any such registration only to the extent that the inclusion of his or its securities will not reduce the amount of Registrable Shares of the Holders which is included in such registration; or (d) otherwise to cause the registration of such person’s or entity’s securities of the Corporation in any manner which are superior to or pari passu with the registration rights granted herein to the Holders.

SECTION 15. Transfer of Registration Rights. The registration rights and obligations of any Holder (and of any permitted transferee or assignee of any Holder or its permitted transferees or assignees) under this Agreement with respect to any Registrable Shares may be transferred to any Affiliate of such Holder or such permitted transferee or assignee, or to any transferee who acquires (otherwise than in a registered public offering) at least five percent (5%) of the Registrable Shares then held by such Holder; provided, however, that (a) the Corporation shall be given written notice by the Holder at the time of any permitted transfer stating the name and address of the transferee or assignee and identifying the securities with respect to which the rights and obligations under this Agreement are being assigned and (b) the transferee or assignee shall execute an agreement to be bound by the terms of this agreement. For purposes of this Section 15, an “Affiliate” of any Holder (or any transferee or assignee of any Holder) means any general or limited partner of any Holder (or transferee or assignee) that is a partnership, any member of any Holder that is an LLC or any person or entity that, directly or indirectly, through one or more intermediaries, controls, or is controlled by, or is under common control with, such Holder or transferee or assignee.

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SECTION 16. Stand-Off Agreement. Each Holder, if requested by the Corporation and the managing underwriter of an offering by the Corporation of Common Stock pursuant to a registration statement under the Securities Act, shall agree not to sell publicly or otherwise transfer or dispose of any Registrable Shares or other securities of the Corporation held by such Holder for a specified period of time (not to exceed 180 days) immediately following the effective date of such registration statement; provided, that:

(a) such agreement shall apply only to the initial public offering of the Corporation’s securities; and

(b) all persons who hold shares of Common Stock, or securities convertible into or exchangeable or exercisable for shares of Common Stock, which in aggregate represent one percent (1%) or more of the shares of Common Stock then outstanding (which 1% shall include all securities convertible into or exchangeable or exercisable for shares of Common Stock, on an as converted, exchanged or exercised basis) (any such person, a “1% Stockholder”), and all officers and directors of the Corporation, enter into similar agreements.

Any discretionary waiver or termination of the restrictions of such agreements (including this Agreement) by the Corporation or the managing underwriter (other than discretionary waivers or releases up to an amount of $50,000 due to financial hardship) shall apply to all persons subject to such agreements on a pro rata basis, based upon the number of shares held by such persons.

SECTION 17. Future Events. The Corporation will notify each Holder participating in a registration of the occurrence of any of the following events of which the Corporation is actually aware, and when so notified, each Holder will immediately discontinue any disposition of Registrable Shares until notified by the Corporation that such event is no longer applicable:

(a) the issuance by the Commission or any state securities commission or agency of any stop order suspending the effectiveness of the registration statement or the initiation of any proceedings for that purpose (in which case the Corporation will make reasonable efforts to obtain the withdrawal of any such order or the cessation of any such proceedings); or

(b) the existence of any fact which makes untrue any material statement made in the registration statement or prospectus or any document incorporated therein by reference or which requires the making of any changes in the registration statement or prospectus or any document incorporated therein by reference in order to make the statements therein not misleading (in which case the Corporation will make reasonable efforts to amend the applicable document to correct the deficiency).

12

SECTION 18. Notices. All notices, requests, consents and other communications hereunder (“Notices”) to any party shall be contained in a written instrument addressed to such party at the address set forth below or such other address as may hereafter be designated in writing by the addressee to the addressor listing all parties and shall be deemed given (a) when delivered in person or duly sent by fax showing confirmation of receipt, (b) three days after being duly sent by first class mail postage prepaid (other than in the case of Notices to or from any non-U.S. resident, which Notices must be sent in the manner specified in clause (a) or (c)), or (c) two days after being duly sent by DHL, Federal Express or other recognized express international courier service:

(a) if to the Corporation, to:

Acceleron Pharma Inc.

128 Sidney Street

Cambridge, MA 02139

with a copy to:

Ropes & Gray LLP

Prudential Tower

800 Boylston Street

Boston, MA 02199-3600

Attn: Marc Rubenstein

Fax: (617) 951-7050

(b) if to the Investors, to their respective addresses set forth on Exhibit A of this Agreement.

SECTION 19. Miscellaneous.

(a) This Agreement states the entire agreement of the parties concerning the subject matter hereof, and supersedes all prior agreements, written or oral, between or among them concerning such subject matter.

(b) This Agreement may be amended, and compliance with any provision of this Agreement may be omitted or waived, only by the written agreement of the Corporation and the Holders of at least two-thirds in voting power of the then outstanding Registrable Shares; provided, however, that no rights of a Holder under this Agreement shall, without its consent, be adversely affected by any such amendment or waiver in any manner in which the rights of other Holders hereunder are not likewise adversely affected.

(c) This Agreement shall be governed by, and construed and enforced in accordance with, the substantive laws of the Commonwealth of Massachusetts, without regard to its principles of conflicts of laws.

(d) This Agreement may be executed in any number of counterparts, each such counterpart shall be deemed to be an original instrument, and all such counterparts together shall constitute but one agreement. Any such counterpart may contain one or more signature pages. This Agreement may be executed by facsimile signature pages.

13

IN WITNESS WHEREOF, the parties have executed this Amended and Restated Registration Rights Agreement as a contract under seal as of the date first written above.

ACCELERON PHARMA INC.
By:	/s/ John Knopf
	John Knopf, Chief Executive Officer

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

POLARIS VENTURE PARTNERS IV, L.P.
BY: POLARIS VENTURE MANAGEMENT CO. IV, L.L.C.         ITS GENERAL PARTNER
POLARIS VENTURE PARTNERS ENTREPRENEURS’ FUND IV, L.P.
BY: POLARIS VENTURE MANAGEMENT CO. IV, L.L.C.         ITS GENERAL PARTNER
By:	/s/ William E. Bilodeau
	William E. Bilodeau Attorney-in-fact

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

VENROCK PARTNERS, L.P.,

by its General Partner, Venrock Partners Management, LLC

VENROCK ASSOCIATES IV, L.P.,

by its General Partner, Venrock Management IV, LLC

VENROCK ENTREPRENEURS FUND IV, L.P.,

by its General Partner, VEF Management IV, LLC

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

ADVANCED TECHNOLOGY VENTURES VII, L.P.		ADVANCED TECHNOLOGY VENTURES VII(C), L.P.
By:	ATV Associates VII, L.L.C., Its General Partner	By:	ATV Associates VII, L.L.C. its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
ADVANCED TECHNOLOGY VENTURES VI, L.P.		ATV ALLIANCE 2003, L.P.
By:	ATV Associates VI, L.L.C., its General Partner	By:	ATV Alliance Associates, L.L.C., Its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
ADVANCED TECHNOLOGY VENTURES VII (B), L.P.		ATV ENTREPRENEURS VI, L.P.
By:	ATV Associates VII, L.L.C., its General Partner	By:	ATV Associates VI, L.L.C., its General Partner
By:	/s/ Jean George	By:	/s/ Jean George
Name:	Jean George	Name:	Jean George
Title:	Managing Director	Title:	Managing Director
ATV ENTREPRENEURS VII, L.P.
By:	ATV Associates VII, L.L.C., its General Partner
By:	/s/ Jean George
Name:	Jean George
Title:	Managing Director

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22 day of December, 2011.

CELGENE CORPORATION
By:	/s/ Perry Karsen
Name:	Perry Karsen
Title:	Chief Operations Officer

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

ORBIMED PRIVATE INVESTMENTS II, LP By:  Orbimed Capital GP II LLC         its General Partner		ORBIMED PRIVATE INVESTMENTS II (QP), LP By:  Orbimed Capital GP II LLC         its General Partner
By:	/s/ Carl Gordon	By:	/s/ Carl Gordon
Name:	Carl Gordon	Name:	Carl Gordon
Title:	Member	Title:	Member
UBS JUNIPER CROSSOVER FUND, L.L.C. By:  Orbimed Advisors LLC         its Member
By:	/s/ Carl Gordon
Name:	Carl Gordon
Title:	Member

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

APPLIED GENOMIC TECHNOLOGY CAPITAL

FUND, L.P.; AGTC ADVISORS FUND, L.P.

Each by: AGTC Partners, L.P., its General Partner

By: NewcoGen Group Inc., its General Partner

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

BESSEMER VENTURE PARTNERS VII L.P.,

BESSEMER VENTURE PARTNERS VII

INSTITUTIONAL L.P.

By: Deer VII & Co. L.P., their General Partner

By: Deer VII & Co. Ltd., its General Partner

By: /s/ Edmund Colloton

Name: Edmund Colloton

Title: Director

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

ALKERMES, INC.
By:	/s/ Michael Landine
Name:	MICHAEL LANDINE
Title:	SENIOR VICE PRESIDENT

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

SUTTER HILL VENTURES, A CALIFORNIA LIMITED PARTNERSHIP		DAVID L. ANDERSON, TRUSTEE OF THE ANDERSON LIVING TRUST U/A/D 1/22/98
By:	Sutter Hill Ventures, L.L.C. its General Partner
By:	/s/ Jeffrey W. Bird	By:	/s/ Robert Yin
Name: Title:	Jeffrey W. Bird Managing Director		David L. Anderson, Trustee By Robert Yin Under Power of Attorney
ANVEST, L.P.		G. LEONARD BAKER, JR. AND MARY ANNE BAKER, CO-TRUSTEES OF THE BAKER REVOCABLE TRUST U/A/D 2/3/03
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	David L. Anderson, Trustee of The Anderson Living Trust U/A/D 1/22/98, General Partner By Robert Yin Under Power of Attorney		G. Leonard Baker, Jr., Trustee By Robert Yin Under Power of Attorney
SAUNDERS HOLDINGS, L.P.		YOVEST, L.P.
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	G. Leonard Baker, Jr., Trustee of The Baker Revocable Trust U/A/D 2/3/03, General Partner By Robert Yin Under Power of Attorney		William H. Younger, Jr., Trustee of The William H. Younger, Jr. Revocable Trust U/A/D 8/5/09, General Partner By Robert Yin Under Power of Attorney
WILLIAM H. YOUNGER, JR. TRUSTEE, THE WILLIAM H. YOUNGER, JR., REVOCABLE TRUST U/A/D 8/5/2009		DAVID E. SWEET AND ROBIN T. SWEET, AS TRUSTEES OF THE DAVID AND ROBIN SWEET LIVING TRUST, DATED 7/6/04
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	William H. Younger, Jr., Trustee	Name:
	By Robert Yin Under Power of Attorney	Title:	By Robert Yin Under Power of Attorney
ROOSTER PARTNERS, LP		GREGORY P. SANDS AND SARAH J.D. SANDS AS TRUSTEES OF GREGORY P. AND SARAH J.D. SANDS TRUST AGREEMENT DATED 2/24/99
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	Tench Coxe, Trustee of The Coxe Revocable Trust U/A/D 4/23/98, General Partner By Robert Yin Under Power of Attorney		Gregory P. Sands, Trustee By Robert Yin Under Power of Attorney

Signature Page to Registration Rights Agreement

JAMES C. GAITHER, TRUSTEE OF THE GAITHER REVOCABLE TRUST U/A/D 9/28/2000		TALLACK PARTNERS, L.P.
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	James C. Gaither, Trustee		James C. Gaither, Trustee of The Gaither Revocable Trust U/A/D 9/28/2000, General Partner
	By Robert Yin Under Power of Attorney	By Robert Yin Under Power of Attorney RONALD D. BERNAL AND PAMELA M. BERNAL AS TRUSTEES OF THE BERNAL FAMILY TRUST U/D/T 11/3/1995
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	James C. Gaither	Name:
		Title:
	By Robert Yin Under Power of Attorney		By Robert Yin Under Power of Attorney
JAMES N. WHITE AND PATRICIA A. O’BRIEN AS TRUSTEES OF THE WHITE FAMILY TRUST U/A/D 4/3/97		JEFFREY W. BIRD AND CHRISTINA R. BIRD AS TRUSTEES OF JEFFREY W. AND CHRISTINA R. BIRD TRUST AGREEMENT DATED 10/31/00
By:	/s/ Robert Yin	By:	/s/ Jeffrey W. Bird
	James N. White, Trustee By Robert Yin Under Power of Attorney		Jeffrey W. Bird, Trustee
ANDREW T. SHEEHAN AND NICOLE J. SHEEHAN AS TRUSTEES OF SHEEHAN 2003 TRUST		MICHAEL L. SPEISER AND MARY ELIZABETH SPEISER, CO TRUSTEES OF SPEISER TRUST AGREEMENT DATED 7/19/06
By:	/s/ Robert Yin	By:	/s/ Robert Yin
	Andrew T. Sheehan, Trustee By Robert Yin Under Power of Attorney		Michael L. Speiser, Trustee By Robert Yin Under Power of Attorney
MICHAEL I. NAAR AND DIANE J. NAAR AS TRUSTEES OF NAAR FAMILY TRUST U/A/D 12.22.94		PATRICK ANDREW CHEN AND YU-YING CHIU CHEN AS TRUSTEES OF PATRICK AND YING CHEN 2001 LIVING TRUST DATED 3/17/01
By:	/s/ Robert Yin	By:	/s/ Robert Yin
Name:		Name:
Title:		Title:
	By Robert Yin Under Power of Attorney		By Robert Yin Under Power of Attorney
TENCH COXE AND SIMONE OTUS COXE, CO-TRUSTEES OF THE COXE REVOCABLE TRUST U/A/D 4/23/98
By:	/s/ Robert Yin
	Tench Coxe, Trustee By Robert Yin Under Power of Attorney

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Sherryl W. Casella	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Sherryl W. Hossack
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Gregory P. Sands	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Tench Coxe
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Ronald D. Bernal	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Lynne M. Brown (Rollover)
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO David E. Sweet	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO William H. Younger, Jr.
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO David E. Sweet (Rollover)	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Robert Yin
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Lynne B. Graw	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO David L. Anderson
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Lynne B. Graw (Rollover)	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Andrew T. Sheehan
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Diane J. Narr	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Yu-Ying Chen
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Patricia Tom (Post)	Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Patricia Tom (Pre)
Wells Fargo Bank, N.A. FBO SHV Profit Sharing Plan FBO Patricia Tom (Rollover)	Wells Fargo Bank N.A. FBO James N. White Roth IRA
Wells Fargo Bank N.A. FBO Jeffrey W. Bird Roth IRA	Wells Fargo Bank N.A. FBO Gregory P. Sands Roth IRA
Wells Fargo Bank N.A. FBO David E. Sweet Roth IRA

By:	/s/ Thomas M. Thurston
Name:	Thomas M. Thurston
Title:	Vice President

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

QVT FUND LP,		QUINTESSENCE FUND L.P.,
BY: ITS GENERAL PARTNER, QVT ASSOCIATES GP LLC		BY: ITS GENERAL PARTNER, QVT ASSOCIATES GP LLC
By:	/s/ Keith S. Manchester	By:	/s/ Keith S. Manchester
Name:	Keith S. Manchester	Name:	Keith S. Manchester
Title:	Portfolio Manager	Title:	Portfolio Manager

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

AVALON VENTURES VI, LP		AVALON VENTURES VI, GP FUND, LLC
By:	/s/ Douglas Downs	By:	/s/ Douglas Downs
Name:	Douglas Downs	Name:	Douglas Downs
Title:	Authorized Signer & CFO	Title:	Authorized Signer & CFO

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

MIDCAP FINANCIAL, LLC, a Delaware limited liability company
By:	/s/ Luis Viera
Name: Luis Viera
Title: Managing Director

Signature Page to Registration Rights Agreement

ACCELERON PHARMA INC.

Amended and Restated Registration Rights Agreement

Investor Signature Page

By executing this page in the space provided, the undersigned hereby agrees (i) that it is an “Investor” as defined in the Amended and Restated Registration Rights Agreement dated as of December 22, 2011, by and among Acceleron Pharma Inc. and the parties named therein (the “Registration Rights Agreement”), (ii) that it is a party to the Registration Rights Agreement for all purposes and (iii) that it is bound by all terms and conditions of the Registration Rights Agreement.

EXECUTED this 22^nd day of December, 2011.

HERCULES TECHNOLOGY II, L.P.
By:	/s/ K. Nicholas Martitsch
Name: K. Nicholas Martitsch
Title: Associate General Counsel

Signature Page to Registration Rights Agreement

EX-7

Exhibit 7

COMMON STOCK PURCHASE AGREEMENT

THIS COMMON STOCK PURCHASE AGREEMENT (this “Agreement”) is made as of September 24, 2013 by and between Acceleron Pharma Inc., a Delaware corporation (the “Corporation”) and Celgene Corporation, a Delaware corporation (“Celgene”).

WHEREAS, pursuant to the terms of that certain Series C-1 Convertible Preferred Stock Purchase Agreement by and between the Corporation and Celgene, dated as of February 20, 2008, Celgene agreed to purchase a certain number of shares of the Corporation’s Common Stock, $.001 par value per share (the “Common Stock”), upon the consummation of the Corporation’s first underwritten public offering of its Common Stock (the “IPO”) at a purchase price equal to the price per share of Common Stock sold to the public in the IPO (the “IPO Price”) (such IPO closing time and IPO Price as set forth on the cover of the final prospectus filed with the Securities and Exchange Commission).

NOW, THEREFORE, in consideration of the premises and the mutual covenants and agreements herein contained, the parties hereby agree as follows:

SECTION 1. Purchase and Sale of the Common Stock. Subject to the terms and conditions of this Agreement, at the Closing, the Corporation agrees to issue and sell to Celgene the number of shares of Common Stock determined by dividing the Purchase Amount by the IPO Price and rounding to the nearest whole share (the “Celgene Shares”). Celgene agrees to purchase from the Corporation at the Closing the Celgene Shares for an aggregate purchase price to be determined by multiplying the total number of Celgene Shares, as determined pursuant to the previous sentence, by the IPO Price. As used in this Section 1, the “Purchase Amount” shall be equal to (i) $10,000,000, if the gross proceeds to the Corporation in the IPO is $50,000,000 or greater; or (ii) twenty percent (20%) of the gross proceeds to the Corporation in the IPO, if such gross proceeds are less than $50,000,000 but greater than or equal to $35,000,000.

SECTION 2. Closing. The closing of the sale and purchase of the Celgene Shares (the “Closing”) shall take place concurrently with the closing of the IPO at the offices of Ropes & Gray LLP, Prudential Tower, 800 Boylston Street, Boston, Massachusetts, or at such other location as may be agreed upon by the Corporation and Celgene, after the satisfaction or waiver of each of the conditions set forth in Section 5 (other than those conditions that by their nature are to be satisfied at the Closing, but subject to the fulfillment or waiver of those conditions). At the Closing, the Corporation shall issue and deliver to Celgene a certificate for shares of Common Stock, registered in the name of Celgene, in the amount representing the number of Celgene Shares, as determined pursuant to Section 1, against payment by Celgene to the Corporation of the aggregate purchase price therefor, as determined pursuant to Section 1, in the form of (a) a certified or bank check payable to the order of the Corporation, (b) a wire transfer to a bank account designated by the Corporation or (c) a combination of (a) and (b).

SECTION 3. Representations and Warranties of the Corporation. The Corporation represents and warrants to Celgene as follows:

3.1 Organization. The Corporation is a corporation duly organized, validly existing and in good standing under the laws of the State of Delaware and has all requisite corporate power and authority to own and lease its properties, to carry on its business as presently conducted and as proposed to be conducted by it and to carry out the transactions contemplated by this Agreement. The Corporation is duly qualified as a foreign corporation and is in good standing in all such jurisdictions in which the conduct of its business or its ownership or leasing of property requires such qualification, except to the extent that any failure to be so qualified would not materially and adversely affect the financial condition, results of operations, assets, liabilities business or prospects of the Corporation.

3.2 Capitalization. Following the conversion of all previously outstanding shares of Preferred Stock of the Corporation into shares of Common Stock of the Corporation and immediately prior to the Closing, the entire authorized capital stock of the Corporation consists of (a) 175,000,000 shares of Common Stock, of which (i) 21,055,604 shares have been issued and are outstanding, fully paid and nonassessable; (ii) no shares are held as treasury shares; (iii) 5,452,965 have been reserved for issuance upon exercise of options granted or to be granted under stock purchase, stock option or other equity incentive plans of the Corporation; (iv) 1,011,590 shares have been reserved for issuance upon exercise of outstanding warrants; and (v) the number of shares determined in accordance with Section 1 are being issued at the Closing and immediately thereafter will be issued and outstanding, fully paid and nonassessable and will be held by Celgene and (b) 25,000,000 shares of Preferred Stock, no shares of which have been issued and are outstanding.

3.3 Authorization of this Agreement. The execution, delivery and performance by the Corporation of this Agreement have been duly authorized by all requisite corporate action. The Corporation has duly authorized, executed and delivered this Agreement, and this Agreement constitutes the valid and binding obligation of the Corporation, enforceable in accordance with its terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)). The execution, delivery and performance of this Agreement, the issuance, sale and delivery of the Celgene Shares, and compliance with the provisions hereof by the Corporation do not and will not, with or without the passage of time or the giving of notice or both, violate, conflict with or result in any breach of any of the terms, conditions or provisions of, or constitute a default (or give rise to any right of termination, cancellation or acceleration) under, or result in the creation of any lien, security interest, charge or encumbrance upon any of the properties or assets of the Corporation, the Certificate of Incorporation or Bylaws of the Corporation, or any provision of law, statute, rule or regulation or any ruling, writ, injunction, order, judgment or decree of any court naming the Corporation, or administrative agency or other governmental body.

3.4 Authorization of the Celgene Shares. The issuance, sale and delivery hereunder by the Corporation of the Celgene Shares have been duly authorized by all requisite corporate action of the Corporation, and when so issued, sold and delivered the Celgene Shares will be validly issued and outstanding, fully paid and nonassessable, and not subject to preemptive or any other similar rights of the stockholders of the Corporation or others.

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3.5 No Governmental Consent or Approval Required. No authorization, consent, approval or other order of, declaration to, or filing with, any governmental agency or body is required to be made or obtained by the Corporation for or in connection with the valid and lawful authorization, execution and delivery by the Corporation of this Agreement or for or in connection with the valid and lawful authorization, issuance, sale and delivery of the Celgene Shares, except exemptive filings under applicable securities laws, which are not required to be made until after the Closing and which shall be made on a timely basis.

3.6 Registration Statement. The Registration Statement and any prospectus contained therein will not, as of the filing date of such Registration Statement and the date of the prospectus, contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading. “Registration Statement” means the registration statement on Form S-1 (File No. 333-190417), including any prospectus filed pursuant to Rule 424 under the Securities Act, and any free writing prospectuses, relating to the IPO.

3.7 Non-Contravention. The Corporation is not in violation or default in any material respect of any provision of its certificate of incorporation or bylaws, or of any instrument, judgment, order, writ or decree to which it is a party or by which it is bound, or, to its knowledge, of any provision of any federal or state statute, rule or regulation applicable to the Corporation, except for such violations or defaults of any federal or state statute, rule or regulation that could not reasonably be expected to result, either individually or in the aggregate, in a material adverse effect on the Corporation’s financial condition, business or operations. The execution, delivery and performance of this Agreement and the consummation of the transactions contemplated hereby will not result in any such violation or constitute, with or without the passage of time and giving of notice, either (i) a default in any material respect of any such instrument, judgment, order, writ or decree or (ii) an event that results in the creation of any lien, charge or encumbrance upon any assets of the Corporation or the suspension, revocation, impairment, forfeiture, or nonrenewal of any material permit, license, authorization or approval applicable to the Corporation, in each case, which could reasonably be expected to result, either individually or in the aggregate, in a material adverse effect on the Corporation’s financial condition, business or operations.

SECTION 4. Representations and Warranties of Celgene. Celgene represents and warrants to the Corporation as follows:

4.1 Purchase for Investment. Celgene is acquiring the Celgene Shares purchasable by it hereunder for its own account, for investment and not for, with a view to, or in connection with, any distribution or public offering thereof within the meaning of the Securities Act of 1933, as amended (the “Securities Act”).

4.2 Unregistered Securities; Legend. Celgene understands that the Celgene Shares have not been, and will not be, registered under the Securities Act or any state securities law, by reason of their issuance in a transaction exempt from the registration requirements of the Securities Act and such laws, that the Celgene Shares must be held indefinitely unless they are subsequently registered under the Securities Act and such laws or a subsequent disposition thereof is exempt from registration, that the certificates for the Celgene Shares shall bear a legend to such effect, and that appropriate stop transfer instructions may be issued. Celgene further understands that such exemption depends upon, among other things, the bona fide nature of Celgene’s investment intent expressed herein.

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4.3 Status of Investor. Celgene has not been formed for the specific purpose of acquiring the Celgene Shares pursuant to this Agreement. Celgene understands the term “accredited investor” as used in Regulation D promulgated under the Securities Act and represents and warrants to the Corporation that Celgene is an “accredited investor” for purposes of acquiring the Celgene Shares purchasable by it hereunder.

4.4 Knowledge and Experience; Economic Risk. Celgene has sufficient knowledge and experience in business and financial matters and with respect to investment in securities of privately held companies so as to enable it to analyze and evaluate the merits and risks of the investment contemplated hereby and is capable of protecting its interest in connection with this transaction. Celgene is able to bear the economic risk of such investment, including a complete loss of the investment.

4.5 Access to Information. Celgene acknowledges that it and its representatives have had the opportunity to ask questions and receive answers from officers and representatives of the Corporation concerning the Corporation and its business and the transactions contemplated by this Agreement and to obtain any additional information which the Corporation possesses or can acquire that is necessary to verify the accuracy of the information regarding the Corporation herein set forth or otherwise desired in connection with its purchase of the Celgene Shares purchasable by it hereunder.

4.6 Rule 144. Celgene understands that the exemption from registration afforded by Rule 144 (the provisions of which are known to Celgene) promulgated by the Securities and Exchange Commission under the Securities Act depends upon the satisfaction of various conditions, and that such exemption is not currently available.

4.7 Place of Business. Celgene has listed its principal place of business or registered address under its name on the signature page hereto.

4.8 Authorization of this Agreement. Celgene has duly authorized, executed and delivered this Agreement, and this Agreement constitutes the valid and binding obligation of Celgene, enforceable against Celgene in accordance with their terms (except as enforceability may be limited by (x) applicable bankruptcy, reorganization, insolvency, moratorium and similar laws affecting the enforcement of creditors’ rights generally and (y) general equitable principles (regardless of whether enforceability is considered in a proceeding in equity or at law)).

SECTION 5. Conditions Precedent to Closing by Celgene. The obligation of Celgene to purchase and pay for the Celgene Shares at the Closing is subject to satisfaction (or waiver by Celgene) of the following conditions precedent at or before the Closing:

5.1 Representations and Warranties Correct. The representations and warranties made by the Corporation in Section 3 hereof shall be true and correct when made, and shall be true and correct in all material respects at the time of the Closing with the same force and effect as if they had been made at and as of the time of the Closing.

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5.2 Closing of IPO. The IPO shall have closed and the IPO results in gross proceeds to the Corporation of not less than $35,000,000 (prior to underwriters’ discounts and commissions) and excluding the proceeds due to the Corporation under this Agreement.

5.3 Qualifications; HSR Clearance. All authorizations, approvals, or permits, if any, of any governmental authority or regulatory body of the United States or of any state that are required in connection with the lawful issuance and sale of the Celgene Shares pursuant to this Agreement shall be duly obtained and effective as of the Closing. If applicable, all waiting periods under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (“HSR Act”) and other applicable antitrust law relating to the transaction contemplated hereby will have expired or terminated early.

SECTION 6. Conditions Precedent to Closing by the Corporation. The obligation of the Corporation to issue and sell the Celgene Shares being sold to Celgene at the Closing is subject to satisfaction (or the waiver by the Corporation) of the following conditions precedent at or before the Closing:

6.1 Representations and Warranties. The representations and warranties made in Section 4 hereof by Celgene shall be true and correct when made, and shall be true and correct in all material respects at the time of the Closing with the same force and effect as if they had been made at and as of the time of the Closing.

6.2 Tender of Payment. At the Closing, Celgene shall have tendered payment for the Celgene Shares being sold at the Closing.

6.3 HSR Clearance. If applicable, all waiting periods under the HSR Act and other applicable antitrust law relating to the transactions contemplated hereby will have expired or terminated early.

SECTION 7. Fees; Brokers.

7.1 Fees and Expenses. Each party to this Agreement shall bear all of its own fees and expenses incurred in connection with the preparation and negotiation of this Agreement and the consummation of the transactions contemplated hereby, including all fees of such party’s legal counsel.

7.2 Brokers. The Corporation represents and warrants to Celgene that (a) neither the Corporation nor any of its officers, directors, employees or stockholders, has employed any broker or finder in connection with the transactions contemplated by this Agreement, and (b) no person or entity will have, as a result of the transactions contemplated by this Agreement, any right to, interest in, or claim against or upon the Corporation or Celgene for, any commission, fee or other compensation as a finder or broker because of any act or omission by the Corporation or any agent of the Corporation.

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SECTION 8. Remedies. In case any one or more of the representations, warranties, covenants or agreements set forth in this Agreement shall have been breached by the Corporation, Celgene may proceed to protect and enforce its rights either by suit in equity or by action at law, including, but not limited to, an action for damages as a result of any such breach or an action for specific performance of any such covenant or agreement contained in this Agreement.

SECTION 9. Indemnification; Limitations on Liability. The Corporation shall indemnify, defend and hold Celgene harmless from and against all liabilities, losses, and damages, together with all reasonable costs and expenses related thereto (including, without limitation, reasonable legal and accounting fees and expenses), which would not have been incurred if (a) all of the representations and warranties of the Corporation in this Agreement had been true and correct when made and at the time of the Closing and (b) all of the covenants and agreements of the Corporation in this Agreement had been duly and timely complied with and performed; provided, however, that the aggregate liability of the Corporation to Celgene under this Section 9 shall not exceed the aggregate purchase price of the Celgene Shares.

SECTION 10. Survival of Representations, Warranties and Agreements. The covenants, representations and warranties of the parties contained herein shall survive any Closing hereunder. Each of the parties may rely on such covenants, representations and warranties irrespective of any investigation made, or notice or knowledge held by, it or any other person.

SECTION 11. Successors and Assigns. This Agreement shall be binding upon, and inure to the benefit of, each of the parties hereto and, except as otherwise expressly provided herein, each other person who shall become a registered holder named in a certificate evidencing Celgene Shares transferred to such holder by Celgene or its permitted transferees, and (except as aforesaid) its legal representatives, successors and assigns.

SECTION 12. Entire Agreement; Effect on Prior Documents. This Agreement and the other documents referred to herein or delivered pursuant hereto contain the entire agreement among the parties with respect to the financing transactions contemplated hereby and supersede all prior negotiations, commitments, agreements and understandings among them with respect thereto.

SECTION 13. Notices. All notices, requests, consents and other communications hereunder (“Notices”) to any party shall be contained in a written instrument addressed to such party at the address set forth below or such other address as may hereafter be designated in writing by the addressee to the addressor listing all parties and shall be deemed given (a) when delivered in person or duly sent by fax showing confirmation of receipt, (b) three days after being duly sent by first class mail postage prepaid (other than in the case of Notices to or from any non-U.S. resident, which Notices must be sent in the manner specified in clause (a) or (c)), or (c) two days after being duly sent by DHL, Federal Express or other recognized express international courier service:

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(a) if to the Corporation, to:

Acceleron Pharma Inc.

128 Sidney Street

Cambridge, MA 02139

with a copy to:

Ropes & Gray LLP

Prudential Tower

800 Boylston Street

Boston, MA 02119-3600

Attn: Marc Rubenstein

Fax: (617) 951-7050

(b) if to Celgene, to:

Celgene Corporation

86 Morris Avenue

Summit, NJ 07901

Attn: Alliance Management

with a copy to:

Celgene Corporation

86 Morris Avenue

Summit, NJ 07901

Attn: Legal Department

Facsimile: (908) 673-2771

SECTION 14. Amendments; Waivers. This Agreement may be amended, and compliance with the provisions of this Agreement may be omitted or waived, only by the written agreement of the Corporation and Celgene.

SECTION 15. Counterparts; Facsimile Signatures. This Agreement may be executed in any number of counterparts, each such counterpart shall be deemed to be an original instrument, and all such counterparts together shall constitute but one agreement. Any such counterpart may contain one or more signature pages. This Agreement may be executed and delivered by facsimile, or by email in portable document format (.pdf) and upon such delivery of the signature page by such method will be deemed to have the same effect as if the original signature had been delivered to the other parties.

SECTION 16. Headings. The headings of the various sections of this Agreement have been inserted for convenience of reference only and shall not be deemed to be a part of this Agreement.

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SECTION 17. Nouns and Pronouns. Whenever the context may require, any pronouns used herein shall include the corresponding masculine, feminine or neuter forms, and the singular form of names and pronouns shall include the plural and vice-versa.

SECTION 18. Governing Law. This Agreement shall be governed by, and construed and enforced in accordance with, the substantive laws of the Commonwealth of Massachusetts, without regard to its principles of conflicts of laws.

SECTION 19. Severability. Any provision of this Agreement that is prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction, be ineffective to the extent of such prohibition or unenforceability without invalidating the remaining provisions hereof, and any such prohibition or unenforceability in any jurisdiction shall not invalidate or render unenforceable such provision in any other jurisdiction.

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IN WITNESS WHEREOF, the undersigned have executed this Common Stock Purchase Agreement as of the day and year first written above.

ACCELERON PHARMA INC.
By:		/s/ John L. Knopf
	Name: John L. Knopf, Ph.D.
	Title: Chief Executive Officer
CELGENE CORPORATION
By:	/s/ Perry Karsen
	Name: Perry Karsen
	Title: Chief Operations Officer
Address:		86 Morris Avenue
		Summit, New Jersery 07901

Signature Page to Common Stock Purchase Agreement