S-8

As filed with the Securities and Exchange Commission on December 12, 2013

Registration No. 333-         

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-8

REGISTRATION STATEMENT
under the
SECURITIES ACT OF 1933

ACCELERON PHARMA INC.

(Exact name of registrant as specified in its charter)

Delaware	27-0072226
(State or other jurisdiction	(I.R.S. Employer
of incorporation or organization)	Identification No.)

128 Sidney Street
Cambridge, Massachusetts 02139

(617) 649-9200
(Address, including Zip Code, of Principal Executive Offices)

Acceleron Pharma Inc. 2003 Stock Option and Restricted Stock Plan
Acceleron Pharma Inc. 2013 Equity Incentive Plan

(Full title of the plan)

John L. Knopf, Ph.D.
Chief Executive Officer and President
128 Sidney Street
Cambridge, Massachusetts 02139

(617) 649-9200
(Name, address and telephone number, including area code, of agent for service)

with copies to:

Marc Rubenstein, Esq.	John D. Quisel, Ph.D., Esq.
Ropes & Gray LLP	Vice President, General Counsel and Secretary
Prudential Tower	Acceleron Pharma Inc.
800 Boylston Street	128 Sidney Street
Boston, Massachusetts 02199	Cambridge, Massachusetts 02139
(617) 951-7000	(617) 649-9200

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Large accelerated filer o Accelerated filer o Non-accelerated filer x
(Do not check if a smaller
reporting company) Smaller reporting company o

CALCULATION OF REGISTRATION FEE

Title of securitiesto be registered		Amount to be registered (1)		Proposed maximumoffering priceper share			Proposed maximum aggregate offeringprice			Amount of registration fee
Acceleron Pharma Inc. 2003 Stock Option and Restricted Stock Plan, $0.001 par value		3,645,255	(2)	$	4.19	(3)	$	15,273,618	(3)	$	1,967
Acceleron Pharma Inc. 2013 Equity Incentive Plan, $0.001 par value		547,750		$	24.11	(4)	$	13,206,253	(4)	$	1,701
Acceleron Pharma Inc. 2013 Equity Incentive Plan, $0.001 par value		952,250		$	24.11	(4)	$	22,958,748		$	2,957
TOTALS		5,145,255					$	51,438,618		$	6,625
(1)	Pursuant to Rule 416(a) of the Securities Act of 1933, as amended, this Registration Statement also covers an indeterminable number of additional shares of Common Stock that may become issuable pursuant to terms designed to prevent dilution resulting from stock splits, stock dividends or similar events.
(2)	Represents shares of Common Stock reserved for issuance upon the exercise of options previously granted under the 2003 Stock Option and Restricted Stock Plan.
(3)	The proposed maximum aggregate offering price for the 2003 Stock Option and Restricted Stock Plan consists of $15,255,600 payable in respect of 3,645,255 shares subject to options at a weighted average exercise price of $4.19 per share.
(4)	This calculation is made solely for the purpose of determining the registration fee pursuant to the provisions of Rule 457(h) under the Securities Act of 1933, as amended.  The proposed maximum aggregate offering price for the 2013 Equity Incentive Plan consists of $13,206,253 payable in respect of 547,750 shares subject to options at an exercise price of $24.11 per share, and the balance of the fee is calculated on the basis of the average high and low prices of Acceleron Pharma Inc. Common Stock, par value $0.001 per share, reported on The NASDAQ Global Market on December 5, 2013.

PART I

INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

Information required by Part I to be contained in the Section 10(a) prospectus is omitted from this Registration Statement in accordance with Rule 428 under the Securities Act of 1933, as amended (the “Securities Act”), and the Note to Part I of Form S-8.

PART II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3. Incorporation of Documents by Reference.

The following documents filed by the Registrant with the Commission are incorporated herein by reference:

(1) The Registrant’s Quarterly Report on Form 10-Q for the period ended September, 30, 2013;

(2) The Registrant’s Current Report on Form 8-K filed September 24, 2013;

(3) The Registrant’s Prospectus filed with the Commission on September 19, 2013 pursuant to Rule 424(b) under the Securities Act relating to the Registrant’s Registration Statement on Form S-1 as amended (File No. 333-190417); and

(4) The description of the Registrant’s Common Stock, $0.001 par value per share, contained in the Registrant’s Registration Statement on Form 8-A, filed with the Commission pursuant to Section 12 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), on September 9, 2013, and any other amendments or reports filed for the purpose of updating such description (File No. 001-36065).

All reports and other documents filed by the Registrant after the date hereof pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act prior to the filing of a post-effective amendment which indicates that all securities offered hereby have been sold or which deregisters all securities then remaining unsold, shall be deemed to be incorporated by reference herein and to be part hereof from the date of filing of such reports and documents. Any statement contained in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded for the purposes of this Registration Statement to the extent that a statement contained herein or in any other subsequently filed document which also is or is deemed to be incorporated by reference herein modifies or supersedes such statement. Any statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this Registration Statement.

Item 4. Description of Securities.

Not applicable.

Item 5. Interests of Named Experts and Counsel.

Not applicable.

Item 6. Indemnification of Directors and Officers.

Section 145 of the General Corporation Law of the State of Delaware provides as follows:

A corporation shall have the power to indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative (other than an action by or in the right of the corporation) by reason of the fact that the person is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by him in connection with such action, suit or proceeding if the person acted in good faith and in a manner the person reasonably believed to be in or not opposed to the best interest of the corporation, and, with respect to any criminal action or proceeding, had no reasonable cause to believe his conduct was unlawful. The termination of any action, suit or proceeding by judgment, order, settlement, conviction or upon a plea of nolo contendere or its equivalent shall not, of itself, create a presumption that the person did not act in good faith and in a

2

manner which the person reasonably believed to be in or not opposed to the best interests of the corporation, and, with respect to any criminal action or proceeding, had reasonable cause to believe that his conduct was unlawful.

A corporation shall have the power to indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action or suit by or in the right of the corporation to procure a judgment in its favor by reason of the fact that the person is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise against expenses (including attorneys’ fees) actually and reasonably incurred by him in connection with the defense or settlement of such action or suit if the person acted in good faith and in a manner the person reasonably believed to be in or not opposed to the best interests of the corporation and except that no indemnification shall be made with respect to any claim, issue or matter as to which such person shall have been adjudged to be liable to the corporation unless and only to the extent that the Court of Chancery or the court in which such action or suit was brought shall determine upon application that, despite the adjudication of liability but in view of all the circumstances of the case, such person is fairly and reasonably entitled to indemnity for such expenses which the Court of Chancery or such other court shall deem proper.

As permitted by the Delaware General Corporation Law, the Registrant’s Seventh Restated Certificate of Incorporation includes a provision to eliminate the personal liability of its directors for monetary damages for breach of their fiduciary duties as directors, subject to certain exceptions. In addition, the Registrant’s Seventh Restated Certificate of Incorporation and Amended and Restated Bylaws provide that the Registrant is required to indemnify its officers and directors under certain circumstances, including those circumstances in which indemnification would otherwise be discretionary, and the Registrant is required to advance expenses to its officers and directors as incurred in connection with proceedings against such officers and directors for which they may be indemnified.

The Registrant has entered into indemnification agreements with its directors and officers. These agreements provide broader indemnity rights than those provided under the Delaware General Corporation Law and under the Registrant’s restated certificate of incorporation and bylaws. The indemnification agreements are not intended to deny or otherwise limit third-party or derivative suits against the Registrant or its directors or officers, but to the extent a director or officer were entitled to indemnity or contribution under the indemnification agreement, the financial burden of a third-party suit would be borne by the Registrant, and the Registrant would not benefit from derivative recoveries against the director or officer. Such recoveries would accrue to the Registrant’s benefit but would be offset by its obligations to the director or officer under the indemnification agreement.

The Registrant maintains directors’ and officers’ liability insurance for the benefit of its directors and officers.

Item 7. Exemption from Registration Claimed.

Not applicable.

Item 8. Exhibits.

Exhibit
Number	Description
4.1	Seventh Restated Certificate of Incorporation of Acceleron Pharma Inc. (previously filed as Exhibit 3.1 to the report on Form 8-K filed September 24, 2013 (File No. 001-36065) and incorporated herein by reference)
4.2	Amended and Restated Bylaws of Acceleron Pharma Inc. (previously filed as Exhibit 3.2 to the report on Form 8-K filed September 24, 2013 (File No. 001-36065) and incorporated herein by reference)
4.3	Form of Common Stock Certificate, $0.001 par value per share (previously filed as Exhibit 4.1 to the registration statement on Form S-1 (File No. 333-190417) and incorporated herein by reference)
4.4	Acceleron Pharma Inc. 2013 Equity Incentive Plan
4.5	Acceleron Pharma Inc. 2003 Stock Option and Restricted Stock Plan (previously filed as Exhibit 10.15 to the registration statement on Form S-1 (File No. 333-190417) and incorporated herein by reference)
5.1	Opinion of Ropes & Gray LLP

3

23.1	Consent of Ropes & Gray LLP (included in Exhibit 5.1)
23.2	Consent of Ernst & Young LLP
24.1	Power of Attorney (included on the signature page in Part II)

Item 9. Undertakings.

(a) The undersigned Registrant hereby undertakes:

(1) To file, during any period in which offers or sales are being made, a post-effective amendment to this Registration Statement:

(i) To include any prospectus required by Section 10(a)(3) of the Securities Act;

(ii) To reflect in the prospectus any facts or events arising after the effective date of the Registration Statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the Registration Statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of a prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than a 20 percent change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement;

(iii) To include any material information with respect to the plan of distribution not previously disclosed in the Registration Statement or any material change to such information in the Registration Statement; provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) above shall not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed with or furnished to the Commission by the Registrant pursuant to Section 13 or 15(d) of the Exchange Act that are incorporated by reference in the Registration Statement.

(2) That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(3) To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(b) The undersigned Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the Registrant’s annual report pursuant to Section 13(a) or 15(d) of the Exchange Act (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference in the Registration Statement shall be deemed to be a new Registration Statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(c) Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the Registrant pursuant to the foregoing provisions, or otherwise, the Registrant has been advised that in the opinion of the Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the Registrant of expenses incurred or paid by a director, officer or controlling person of the Registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the Registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

4

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Cambridge, Commonwealth of Massachusetts on the 11th day of December, 2013.

ACCELERON PHARMA INC.
By:	/s/ John L. Knopf, Ph.D.
	John L. Knopf, Ph.D.
	Chief Executive Officer, President and Director

POWER OF ATTORNEY

Each person whose signature appears below constitutes and appoints John L. Knopf and  Kevin F. McLaughlin, and each of them singly, his true and lawful attorneys-in-fact and agents, with full power of substitution and resubstitution in each of them singly, for him and in his name, place and stead, and in any and all capacities, to sign any and all amendments (including post-effective amendments) to this Registration Statement on Form S-8 of Acceleron Pharma Inc., and to file the same, with all exhibits thereto and other documents in connection therewith, with the Securities and Exchange Commission, granting to the attorneys-in-fact and agents, and each of them, full power and authority to do and perform each and every act and thing requisite or necessary to be done in or about the premises, as full to all intents and purposes as he might or could do in person, hereby ratifying and confirming all that the attorneys-in-fact and agents or any of each of them or their substitutes may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

Signature	Title	Date
/s/ John L. Knopf, Ph.D.	Chief Executive Officer, President and Director	December 11, 2013
John L. Knopf, Ph.D.	(Principal Executive Officer)
/s/ Kevin F. McLaughlin	Chief Financial Officer (Principal Financial Officer and	December 11, 2013
Kevin F. McLaughlin	Principal Accounting Officer)
		December 11, 2013
/s/ Anthony B. Evnin, Ph.D.	Director
Anthony B. Evnin, Ph.D.
/s/ Jean M. George		December 11, 2013
Jean M. George	Director
/s/ Edwin M. Kania, Jr.		December 11, 2013
Edwin M. Kania, Jr.	Director

5

/s/ Terrance G. McGuire		December 11, 2013
Terrance G. McGuire	Director
/s/ Richard F. Pops		December 11, 2013
Richard F. Pops	Director
/s/ Joseph S. Zakrzewski		December 11, 2013
Joseph S. Zakrzewski	Director

6

INDEX OF EXHIBITS

Exhibit
Number	Description
4.1	Seventh Restated Certificate of Incorporation of Acceleron Pharma Inc. (previously filed as Exhibit 3.1 to the report on Form 8-K filed September 24, 2013 (File No. 001-36065) and incorporated herein by reference)
4.2	Amended and Restated Bylaws of Acceleron Pharma Inc. (previously filed as Exhibit 3.2 to the report on Form 8-K filed September 24, 2013 (File No. 001-36065) and incorporated herein by reference)
4.3	Form of Common Stock Certificate, $0.001 par value per share (previously filed as Exhibit 4.1 to the registration statement on Form S-1 (File No. 333-190417) and incorporated herein by reference)
4.4	Acceleron Pharma Inc. 2013 Equity Incentive Plan
4.5	Acceleron Pharma Inc. 2003 Stock Option and Restricted Stock Plan (previously filed as Exhibit 10.15 to the registration statement on Form S-1 (File No. 333-190417) and incorporated herein by reference)
5.1	Opinion of Ropes & Gray LLP
23.1	Consent of Ropes & Gray LLP (included in Exhibit 5.1)
23.2	Consent of Ernst & Young LLP
24.1	Power of Attorney (included on the signature page in Part II)

7

EX-4.4

Exhibit 4.4

ACCELERON PHARMA INC.
2013 EQUITY INCENTIVE PLAN

1.DEFINED TERMS

Exhibit A, which is incorporated by reference, defines the terms used in the Plan and sets forth certain operational rules related to those terms.

2.PURPOSE

The Plan has been established to advance the interests of the Company by providing for the grant to Participants of Stock-based and other incentive Awards.

3.ADMINISTRATION

The Administrator has discretionary authority, subject only to the express provisions of the Plan, to interpret the Plan; determine eligibility for and grant Awards; determine, modify or waive the terms and conditions of any Award; prescribe forms, rules and procedures relating to the Plan; and otherwise do all things necessary or appropriate to carry out the purposes of the Plan.  Determinations of the Administrator made under the Plan will be conclusive and will bind all parties.

4.LIMITS ON AWARDS UNDER THE PLAN

(a)Number of Shares.  The maximum number of shares of Stock that may be delivered in satisfaction of Awards under the Plan is 1,344,116, plus 155,884 shares of Stock that are available for grant under the Acceleron Pharma Inc. 2003 Stock Option and Restricted Stock Plan as of the date of adoption of the Plan (the “Share Pool”).  The Share Pool shall automatically increase annually on each January 1st, from January 1, 2014 through January 1, 2023, in an amount equal to the lesser of (a) 3,150,000 shares, or (b) four percent (4%) of the number of shares of Stock outstanding as of the close of business on the immediately preceding December 31st.    Notwithstanding the foregoing, the Board may act prior to January 1st of a given year to provide that there will be no January 1st increase in the Share Pool for such year or that the increase in the Share Pool for such year will be a lesser number of shares of Stock than would otherwise occur pursuant to the preceding sentence.  Up to the total number of shares set forth in this Section 4(a) as increased by the provisions of this Section 4(a) above may be issued in satisfaction of ISOs, but nothing in this Section 4(a) will be construed as requiring that any, or any fixed number of, ISOs be awarded under the Plan.  The limits set forth in this Section 4(a) shall be construed to comply with Section 422 of the Code.  For purposes of this Section 4(a), the number of shares of Stock delivered in satisfaction of Awards will be determined net of shares of Stock withheld by the Company in payment of the exercise price of the Award or in satisfaction of tax withholding requirements with respect to the Award and, for the avoidance of doubt, without including any share of Stock underlying Awards settled in cash or that otherwise expire or become unexercisable without having been exercised or that are forfeited to or repurchased by the Company due to failure to vest.

(b)Type of Shares.  Stock delivered by the Company under the Plan may be authorized but unissued Stock or previously issued Stock acquired by the Company.  No fractional shares of Stock will be delivered under the Plan.

(c)Section 162(m) Limits.  The following additional limits will apply to Awards of the specified type granted to any person in any calendar year:

(1)Stock Options:  1,000,000 shares of Stock.

(2)SARs:  1,000,000 shares of Stock.

(3)Awards other than Stock Options or SARs:  500,000 shares of Stock.

In applying the foregoing limits, (i) all Awards of the specified type granted to the same person in the same calendar year will be aggregated and made subject to one limit; (ii) the limits applicable to Stock Options and SARs refer to the number of shares of Stock subject to those Awards; and (iii) the share limit under clause (3) refers to the maximum number of shares of Stock that may be delivered, or the value of which could be paid in cash or other property, under an Award or Awards of the type specified in clause (3) assuming a maximum payout.  The foregoing provisions will be construed in a manner consistent with Section 162(m), including, without limitation, where applicable, the rules under Section 162(m) pertaining to permissible deferrals of exempt awards.

5.ELIGIBILITY AND PARTICIPATION

The Administrator will select Participants from among key Employees and directors of, and consultants and advisors to, the Company and its Affiliates who are in a position to contribute significantly to the success of the Company and its Affiliates.  Eligibility for ISOs is limited to individuals described in the first sentence of this Section 5 who are employees of the Company or of a “parent corporation” or “subsidiary corporation” of the Company as those terms are defined in Section 424 of the Code.  Eligibility for Stock Options other than ISOs is limited to individuals described in the first sentence of this Section 5 who are providing direct services on the date of grant of the Stock Option to the Company or to a subsidiary of the Company that would be described in the first sentence of Treas. Regs. §1.409A-1(b)(5)(iii)(E).

6.RULES APPLICABLE TO AWARDS

(a)All Awards.

(1)Award Provisions.  The Administrator will determine the terms of all Awards, subject to the limitations provided herein.  By accepting (or, under such rules as the Administrator may prescribe, being deemed to have accepted) an Award, the Participant will be deemed to have agreed to the terms of the Award and the Plan.  Notwithstanding any provision of this Plan to the contrary, awards of an acquired company that are converted, replaced or adjusted in connection with the acquisition may contain terms and conditions that are inconsistent with the terms and conditions specified herein, as determined by the Administrator.

2

(2)Term of Plan.  No Awards may be made after ten years from the Date of Adoption, but previously granted Awards may continue beyond that date in accordance with their terms.

(3)Transferability.  Neither ISOs nor, except as the Administrator otherwise expressly provides in accordance with the third sentence of this Section 6(a)(3), other Awards may be transferred other than by will or by the laws of descent and distribution.  During a Participant’s lifetime, ISOs (and, except as the Administrator otherwise expressly provides in accordance with the third sentence of this Section 6(a)(3), SARs and NSOs) may be exercised only by the Participant.  The Administrator may permit the gratuitous transfer (i.e., transfer not for value) of Awards other than ISOs, subject to such limitations as the Administrator may impose.

(4)Vesting, etc.  The Administrator will determine the time or times at which an Award will vest or become exercisable and the terms on which a Stock Option or SAR will remain exercisable.  Without limiting the foregoing, the Administrator may at any time accelerate the vesting or exercisability of an Award, regardless of any adverse or potentially adverse tax or other consequences resulting from such acceleration.  Unless the Administrator expressly provides otherwise, however, the following rules will apply if a Participant’s Employment ceases:

(A)Immediately upon the cessation of the Participant’s Employment and except as provided in (B) and (C) below, each Stock Option and SAR that is then held by the Participant or by the Participant’s permitted transferees, if any, will cease to be exercisable and will terminate and all other Awards that are then held by the Participant or by the Participant’s permitted transferees, if any, to the extent not already vested will be forfeited.

(B)Subject to (C) and (D) below, all Stock Options and SARs held by the Participant or the Participant’s permitted transferees, if any, immediately prior to the cessation of the Participant’s Employment, to the extent then exercisable, will remain exercisable for the lesser of (i) a period of three months or (ii) the period ending on the latest date on which such Stock Option or SAR could have been exercised without regard to this Section 6(a)(4), and will thereupon immediately terminate.

(C)All Stock Options and SARs held by a Participant or the Participant’s permitted transferees, if any, immediately prior to the cessation of the Participant’s Employment due to his or her death, to the extent then exercisable, will remain exercisable for the lesser of (i) a period of twelve (12) months or (ii) the period ending on the latest date on which such Stock Option or SAR could have been exercised without regard to this Section 6(a)(4), and will thereupon immediately terminate.

(D)All Stock Options and SARs (whether or not exercisable) held by a Participant or the Participant’s permitted transferees, if any, immediately prior to the cessation of the Participant’s Employment will immediately terminate upon such cessation of Employment if the termination is for Cause or occurs in circumstances that

3

in the sole determination of the Administrator would have constituted grounds for the Participant’s Employment to be terminated for Cause.

(5)Additional Restrictions.  The Administrator may cancel, rescind, withhold or otherwise limit or restrict any Award at any time if the Participant is not in compliance with all applicable provisions of the Award agreement and the Plan, or if the Participant breaches any agreement with the Company or its Affiliates with respect to non-competition, non-solicitation or confidentiality.  Without limiting the generality of the foregoing, the Administrator may recover Awards made under the Plan and payments under or gain in respect of any Award in accordance with any applicable Company clawback or recoupment policy, as such policy may be amended and in effect from time to time, or as otherwise required by law or applicable stock exchange listing standards, including, without limitation, Section 10D of the Securities Exchange Act of 1934, as amended.

(6)Taxes.  The delivery, vesting and retention of Stock, cash or other property under an Award are conditioned upon full satisfaction by the Participant of all tax withholding requirements with respect to the Award.  The Administrator will prescribe such rules for the withholding of taxes as it deems necessary.  The Administrator may, but need not, hold back shares of Stock from an Award or permit a Participant to tender previously owned shares of Stock in satisfaction of tax withholding requirements (but not in excess of the minimum withholding required by law).

(7)Dividend Equivalents, Etc.  The Administrator may provide for the payment of amounts (on terms and subject to conditions established by the Administrator) in lieu of cash dividends or other cash distributions with respect to Stock subject to an Award whether or not the holder of such Award is otherwise entitled to share in the actual dividend or distribution in respect of such Award.  Any entitlement to dividend equivalents or similar entitlements will be established and administered either consistent with an exemption from, or in compliance with, the requirements of Section 409A.  Dividends or dividend equivalent amounts payable in respect of Awards that are subject to restrictions may be subject to such limits or restrictions as the Administrator may impose.

(8)Rights Limited.  Nothing in the Plan will be construed as giving any person the right to continued employment or service with the Company or its Affiliates, or any rights as a stockholder except as to shares of Stock actually issued under the Plan.  The loss of existing or potential profit in Awards will not constitute an element of damages in the event of termination of Employment for any reason, even if the termination is in violation of an obligation of the Company or any Affiliate to the Participant.

(9)Section 162(m).  In the case of any Performance Award (other than a Stock Option or SAR) intended to qualify for the performance-based compensation exception under Section 162(m), the Administrator will establish the applicable Performance Criterion or Criteria in writing no later than ninety (90) days after the commencement of the period of service to which the performance relates (or at such earlier time as is required to qualify the Award as performance-based under Section 162(m)) and, prior to the event or occurrence (grant, vesting or payment, as the case may be) that is conditioned on the attainment of such Performance Criterion or Criteria, will certify whether it or they have been attained.  The preceding sentence will not

4

apply to an Award eligible (as determined by the Administrator) for exemption from the limitations of Section 162(m) by reason of the post-initial public offering transition relief in Section 1.162-27(f) of the Treasury Regulations.

(10)Coordination with Other Plans.  Awards under the Plan may be granted in tandem with, or in satisfaction of or substitution for, other Awards under the Plan or awards made under other compensatory plans or programs of the Company or its Affiliates.  For example, but without limiting the generality of the foregoing, awards under other compensatory plans or programs of the Company or its Affiliates may be settled in Stock (including, without limitation, Unrestricted Stock) if the Administrator so determines, in which case the shares delivered will be treated as awarded under the Plan (and will reduce the number of shares thereafter available under the Plan in accordance with the rules set forth in Section 4).  In any case where an award is made under another plan or program of the Company or its Affiliates and such award is intended to qualify for the performance-based compensation exception under Section 162(m), and such award is settled by the delivery of Stock or another Award under the Plan, the applicable Section 162(m) limitations under both the other plan or program and under the Plan will be applied to the Plan as necessary (as determined by the Administrator) to preserve the availability of the Section 162(m) performance-based compensation exception with respect thereto.

(11)Section 409A.  Each Award will contain such terms as the Administrator determines, and will be construed and administered, such that the Award either qualifies for an exemption from the requirements of Section 409A or satisfies such requirements.

(12)Fair Market Value.  In determining the fair market value of any share of Stock under the Plan, the Administrator will make the determination in good faith consistent with the rules of Section 422 and Section 409A to the extent applicable.

(b)Stock Options and SARs.

(1)Time And Manner Of Exercise.  Unless the Administrator expressly provides otherwise, no Stock Option or SAR will be deemed to have been exercised until the Administrator receives a notice of exercise (in form acceptable to the Administrator), which may be an electronic notice, signed (including electronic signature in form acceptable to the Administrator) by the appropriate person and accompanied by any payment required under the Award.  A Stock Option or SAR exercised by any person other than the Participant will not be deemed to have been exercised until the Administrator has received such evidence as it may require that the person exercising the Award has the right to do so.

(2)Exercise Price.  The exercise price (or the base value from which appreciation is to be measured) of each Award requiring exercise will be no less than 100% (or in the case of an ISO granted to a ten-percent shareholder within the meaning of subsection (b)(6) of Section 422, 110%) of the fair market value of the Stock subject to the Award, determined as of the date of grant, or such higher amount as the Administrator may determine in connection with the grant.  Except in connection with a corporate transaction involving the Company (which term shall include, without limitation, any stock dividend, stock split, extraordinary cash dividend, recapitalization, reorganization, merger, consolidation, split-up,

5

spin-off, combination, or exchange of shares), the terms of outstanding Awards may not be amended to reduce the exercise prices of outstanding Stock Options or the base values from which appreciation under outstanding SARs are to be measured other than in accordance with the stockholder approval requirements of the NASDAQ Global Market.

(3)Payment Of Exercise Price.  Where the exercise of an Award is to be accompanied by payment, payment of the exercise price will be by cash or check acceptable to the Administrator or by such other legally permissible means, if any, as may be acceptable to the Administrator.

(4)Maximum Term.  Stock Options and SARs will have a maximum term not to exceed ten (10) years from the date of grant (or five (5) years from the date of grant in the case of an ISO granted to a ten-percent shareholder described in Section 6(b)(2) above); provided, however, that, if a Participant still holding an outstanding but unexercised NSO or SAR ten (10) years from the date of grant (or, in the case of an NSO or SAR with a maximum term of less than ten (10) years, such maximum term) is prohibited by applicable law or a written policy of the Company applicable to similarly situated employees from engaging in any open-market sales of Stock, and if at such time the Stock is publicly traded (as determined by the Administrator), the maximum term of such Award will instead be deemed to expire on the thirtieth (30th) day following the date the Participant is no longer prohibited from engaging in such open market sales.

7.EFFECT OF CERTAIN TRANSACTIONS

(a)Mergers, etc.  Except as otherwise provided in an Award agreement, the following provisions will apply in the event of a Covered Transaction:

(1)Assumption or Substitution.  If the Covered Transaction is one in which there is an acquiring or surviving entity, the Administrator may (but, for the avoidance of doubt, need not) provide (i) for the assumption or continuation of some or all outstanding Awards or any portion thereof or (ii) for the grant of new awards in substitution therefor by the acquiror or survivor or an affiliate of the acquiror or survivor.

(2)Cash-Out of Awards.  Subject to Section 7(a)(5) below the Administrator may (but, for the avoidance of doubt, need not) provide for payment (a “cash-out”), with respect to some or all Awards or any portion thereof, equal in the case of each affected Award or portion thereof to the excess, if any, of (A) the fair market value of one share of Stock (as determined by the Administrator in its reasonable discretion) times the number of shares of Stock subject to the Award or such portion, over (B) the aggregate exercise or purchase price, if any, under the Award or such portion (in the case of an SAR, the aggregate base value above which appreciation is measured), in each case on such payment terms (which need not be the same as the terms of payment to holders of Stock) and other terms, and subject to such conditions, as the Administrator determines, it being understood that if the exercise or purchase price (or base value) of an Award is equal to or greater than the fair market value of one share of Stock, the Award may be cancelled with no payment due hereunder.

6

(3)Acceleration of Certain Awards.  Subject to Section 7(a)(5) below, the Administrator may (but, for the avoidance of doubt, need not) provide that any Award requiring exercise will become exercisable, in full or in part and/or that the delivery of any shares of Stock remaining deliverable under any outstanding Award of Stock Units (including Restricted Stock Units and Performance Awards to the extent consisting of Stock Units) will be accelerated in full or in part, in each case on a basis that gives the holder of the Award a reasonable opportunity, as determined by the Administrator, following exercise of the Award or the delivery of the shares, as the case may be, to participate as a stockholder in the Covered Transaction.

(4)Termination of Awards Upon Consummation of Covered Transaction.  Except as the Administrator may otherwise determine in any case, each Award will automatically terminate (and in the case of outstanding shares of Restricted Stock, will automatically be forfeited) upon consummation of the Covered Transaction, other than Awards assumed pursuant to Section 7(a)(1) above.

(5)Additional Limitations.  Any share of Stock and any cash or other property delivered pursuant to Section 7(a)(2) or Section 7(a)(3) above with respect to an Award may, in the discretion of the Administrator, contain such restrictions, if any, as the Administrator deems appropriate to reflect any performance or other vesting conditions to which the Award was subject and that did not lapse (and were not satisfied) in connection with the Covered Transaction.  For purposes of the immediately preceding sentence, a cash-out under Section 7(a)(2) above or acceleration under Section 7(a)(3) above will not, in and of itself, be treated as the lapsing (or satisfaction) of a performance or other vesting condition.  In the case of Restricted Stock that does not vest and is not forfeited in connection with the Covered Transaction, the Administrator may require that any amounts delivered, exchanged or otherwise paid in respect of such Stock in connection with the Covered Transaction be placed in escrow or otherwise made subject to such restrictions as the Administrator deems appropriate to carry out the intent of the Plan.

(b)Changes in and Distributions With Respect to Stock.

(1)Basic Adjustment Provisions.  In the event of a stock dividend, stock split or combination of shares (including a reverse stock split), recapitalization or other change in the Company’s capital structure that constitutes an equity restructuring within the meaning of FASB ASC 718, the Administrator will make appropriate adjustments to the Share Pool and to the maximum share limits described in Section 4(c), and will also make appropriate adjustments to the number and kind of shares of stock or securities subject to Awards then outstanding or subsequently granted, any exercise or purchase prices (or base values) relating to Awards and any other provision of Awards affected by such change.

(2)Certain Other Adjustments.  The Administrator may also make adjustments of the type described in Section 7(b)(1) above to take into account distributions to stockholders other than those provided for in Section 7(a) and 7(b)(1), or any other event, if the Administrator determines that adjustments are appropriate to avoid distortion in the operation of the Plan, having due regard for the qualification of ISOs under Section 422, the requirements of Section 409A, and for the performance-based compensation rules of Section 162(m), where applicable.

7

(3)Continuing Application of Plan Terms.  References in the Plan to shares of Stock will be construed to include any stock or securities resulting from an adjustment pursuant to this Section 7.

8.LEGAL CONDITIONS ON DELIVERY OF STOCK

The Company will not be obligated to deliver any shares of Stock pursuant to the Plan or to remove any restriction from shares of Stock previously delivered under the Plan until: (i) the Company is satisfied that all legal matters in connection with the issuance and delivery of such shares have been addressed and resolved; (ii) if the outstanding Stock is at the time of delivery listed on any stock exchange or national market system, the shares to be delivered have been listed or authorized to be listed on such exchange or system upon official notice of issuance; and (iii) all conditions of the Award have been satisfied or waived.  The Company may require, as a condition to exercise of the Award, such representations or agreements as counsel for the Company may consider appropriate to avoid violation of the Securities Act of 1933, as amended, or any applicable state or non-U.S. securities law.  Any Stock required to be issued to Participants under the Plan will be evidenced in such manner as the Administrator may deem appropriate, including book-entry registration or delivery of stock certificates.  In the event that the Administrator determines that Stock certificates will be issued to Participants under the Plan, the Administrator may require that certificates evidencing Stock issued under the Plan bear an appropriate legend reflecting any restriction on transfer applicable to such Stock, and the Company may hold the certificates pending lapse of the applicable restrictions.

9.AMENDMENT AND TERMINATION

The Administrator may at any time or times amend the Plan or any outstanding Award for any purpose which may at the time be permitted by law, and may at any time terminate the Plan as to any future grants of Awards; provided, that, except as otherwise expressly provided in the Plan, the Administrator may not, without the Participant’s consent, alter the terms of an Award so as to affect materially and adversely the Participant’s rights under the Award, unless the Administrator expressly reserved the right to do so at the time the Award was granted.  Any amendments to the Plan will be conditioned upon stockholder approval only to the extent, if any, such approval is required by law (including the Code and applicable stock exchange requirements), as determined by the Administrator.

10.OTHER COMPENSATION ARRANGEMENTS

The existence of the Plan or the grant of any Award will not in any way affect the Company’s right to Award a person bonuses or other compensation in addition to Awards under the Plan.

11.MISCELLANEOUS

(a)Waiver of Jury Trial.  By accepting an Award under the Plan, each Participant waives any right to a trial by jury in any action, proceeding or counterclaim concerning any rights under the Plan and any Award, or under any amendment, waiver, consent, instrument, document or other agreement delivered or which in the future may be delivered in connection therewith, and agrees that any such action, proceedings or counterclaim will be tried before a

8

court and not before a jury.  By accepting an Award under the Plan, each Participant certifies that no officer, representative, or attorney of the Company has represented, expressly or otherwise, that the Company would not, in the event of any action, proceeding or counterclaim, seek to enforce the foregoing waivers.  Notwithstanding anything to the contrary in the Plan, nothing herein is to be construed as limiting the ability of the Company and a Participant to agree to submit disputes arising under the terms of the Plan or any Award made hereunder to binding arbitration or as limiting the ability of the Company to require any eligible individual to agree to submit such disputes to binding arbitration as a condition of receiving an Award hereunder.

(b)Limitation of Liability.  Notwithstanding anything to the contrary in the Plan, neither the Company, nor any Affiliate, nor the Administrator, nor any person acting on behalf of the Company, any Affiliate, or the Administrator, will be liable to any Participant or to the estate or beneficiary of any Participant or to any other holder of an Award by reason of any acceleration of income, or any additional tax (including any interest and penalties), asserted by reason of the failure of an Award to satisfy the requirements of Section 422 or Section 409A or by reason of Section 4999 of the Code, or otherwise asserted with respect to the Award; provided, that nothing in this Section 11(b) will limit the ability of the Administrator or the Company, in its discretion, to provide by separate express written agreement with a Participant for any payment in connection with any such acceleration of income or additional tax.

12.ESTABLISHMENT OF SUB-PLANS

The Administrator may from time to time establish one or more sub-plans under the Plan for purposes of satisfying applicable blue sky, securities or tax laws of various jurisdictions.  The Administrator will establish such sub-plans by adopting supplements to the Plan setting forth (i) such limitations on the Administrator’s discretion under the Plan as it deems necessary or desirable and (ii) such additional terms and conditions not otherwise inconsistent with the Plan as it deems necessary or desirable.  All supplements so established will be deemed to be part of the Plan, but each supplement will apply only to Participants within the affected jurisdiction (as determined by the Administrator).

13.GOVERNING LAW

(a)Certain Requirements of Corporate Law.  Awards will be granted and administered consistent with the requirements of applicable Delaware law relating to the issuance of stock and the consideration to be received therefor, and with the applicable requirements of the stock exchanges or other trading systems on which the Stock is listed or entered for trading, in each case as determined by the Administrator.

(b)Other Matters.  Except as otherwise provided by the express terms of an Award agreement, under a sub-plan described in Section 12 or as provided in Section 13(a) above, the provisions of the Plan and of Awards under the Plan and all claims or disputes arising out of our based upon the Plan or any Award under the Plan or relating to the subject matter hereof or thereof will be governed by and construed in accordance with the domestic substantive laws of the State of Massachusetts without giving effect to any choice or conflict of laws provision or rule that would cause the application of the domestic substantive laws of any other jurisdiction.

9

(c)Jurisdiction.  By accepting an Award, each Participant will be deemed to (a) have submitted irrevocably and unconditionally to the jurisdiction of the federal and state courts located within the geographic boundaries of the United States District Court for the District of Massachusetts for the purpose of any suit, action or other proceeding arising out of or based upon the Plan or any Award; (b) agree not to commence any suit, action or other proceeding arising out of or based upon the Plan or an Award, except in the federal and state courts located within the geographic boundaries of the United States District Court for the District of Massachusetts; and (c) waive, and agree not to assert, by way of motion as a defense or otherwise, in any such suit, action or proceeding, any claim that it is not subject personally to the jurisdiction of the above-named courts that its property is exempt or immune from attachment or execution, that the suit, action or proceeding is brought in an inconvenient forum, that the venue of the suit, action or proceeding is improper or that the Plan or an Award or the subject matter thereof may not be enforced in or by such court.

10

EXHIBIT A

Definition of Terms

The following terms, when used in the Plan, will have the meanings and be subject to the provisions set forth below:

“Administrator”:  The Compensation Committee, except that the Compensation Committee may delegate (i) to one or more of its members (or one or more other members of the Board (including the full Board)) such of its duties, powers and responsibilities as it may determine; (ii) to one or more officers of the Company the power to grant Awards to the extent permitted by Section 157(c) of the Delaware General Corporation Law; and (iii) to such Employees or other persons as it determines such ministerial tasks as it deems appropriate.  In the event of any delegation described in the preceding sentence, the term “Administrator” will include the person or persons so delegated to the extent of such delegation.

“Affiliate”:  Any corporation or other entity that stands in a relationship to the Company that would result in the Company and such corporation or other entity being treated as one employer under Section 414(b) and Section 414(c) of the Code.

“Award”:  Any or a combination of the following:

(i) Stock Options.

(ii) SARs.

(iii) Restricted Stock.

(iv) Unrestricted Stock.

(v)  Stock Units, including Restricted Stock Units.

(vi) Performance Awards.

(vii)  Awards (other than Awards described in (i) through (vi) above) that are convertible into or otherwise based on Stock.

“Board”:  The Board of Directors of the Company.

“Cause”:  In the case of any Participant who is party to an employment or severance-benefit agreement that contains a definition of “Cause,” the definition set forth in such agreement will apply with respect to such Participant under the Plan for so long as such agreement is in effect.  In the case of any other Participant, “Cause” will mean, as determined by the Administrator in its reasonable judgment, (i) a substantial failure of the Participant to perform the Participant’s duties and responsibilities to the Company or subsidiaries or substantial negligence in the performance of such duties and responsibilities; (ii) the commission by the Participant of a felony or a crime involving moral turpitude; (iii) the commission by the Participant of theft, fraud, embezzlement, material breach of trust or any material act of

11

dishonesty involving the Company or any of its subsidiaries; (iv) a significant violation by the Participant of the code of conduct of the Company or its subsidiaries of any material policy of the Company or its subsidiaries, or of any statutory or common law duty of loyalty to the Company or its subsidiaries; (v) material breach of any of the terms of the Plan or any Award made under the Plan, or of the terms of any other agreement between the Company or subsidiaries and the Participant; or (vi) other conduct by the Participant that could reasonably be expected to be harmful to the business, interests or reputation of the Company.

“Code”:  The U.S. Internal Revenue Code of 1986 as from time to time amended and in effect, or any successor statute as from time to time in effect.

“Compensation Committee”:  The Compensation Committee of the Board.

“Company”:  Acceleron Pharma Inc.

“Covered Transaction”:  Any of (i) a consolidation, merger, or similar transaction or series of related transactions, including a sale or other disposition of stock, in which the Company is not the surviving corporation or which results in the acquisition of all or substantially all of the Company’s then outstanding common stock by a single person or entity or by a group of persons and/or entities acting in concert, (ii) a sale or transfer of all or substantially all the Company’s assets, or (iii) a dissolution or liquidation of the Company.  Where a Covered Transaction involves a tender offer that is reasonably expected to be followed by a merger described in clause (i) (as determined by the Administrator), the Covered Transaction will be deemed to have occurred upon consummation of the tender offer.

“Date of Adoption”:  The earlier of the date the Plan was approved by the Company’s stockholders or adopted by the Board, as determined by the Compensation Committee.

“Employee”:  Any person who is employed by the Company or an Affiliate.

“Employment”:  A Participant’s employment or other service relationship with the Company and its Affiliates.  Employment will be deemed to continue, unless the Administrator expressly provides otherwise, so long as the Participant is employed by, or otherwise is providing services in a capacity described in Section 5 to the Company or an Affiliate.  If a Participant’s employment or other service relationship is with an Affiliate and that entity ceases to be an Affiliate, the Participant’s Employment will be deemed to have terminated when the entity ceases to be an Affiliate unless the Participant transfers Employment to the Company or its remaining Affiliates.  Notwithstanding the foregoing and the definition of “Affiliate” above, in construing the provisions of any Award relating to the payment of “nonqualified deferred compensation” (subject to Section 409A) upon a termination or cessation of Employment, references to termination or cessation of employment, separation from service, retirement or similar or correlative terms will be construed to require a “separation from service” (as that term is defined in Section 1.409A-1(h) of the Treasury Regulations) from the Company and from all other corporations and trades or businesses, if any, that would be treated as a single “service recipient” with the Company under Section 1.409A-1(h)(3) of the Treasury Regulations.  The Company may, but need not, elect in writing, subject to the applicable limitations under Section 409A, any of the special elective rules prescribed in Section 1.409A-1(h) of the Treasury

12

Regulations for purposes of determining whether a “separation from service” has occurred.  Any such written election will be deemed a part of the Plan.

“ISO”:  A Stock Option intended to be an “incentive stock option” within the meaning of Section 422.  Each Stock Option granted pursuant to the Plan will be treated as providing by its terms that it is to be an NSO unless, as of the date of grant, it is expressly designated as an ISO.

“NSO”:  A Stock Option that is not intended to be an “incentive stock option” within the meaning of Section 422.

“Participant”:  A person who is granted an Award under the Plan.

“Performance Award”:  An Award subject to Performance Criteria.  The Administrator in its discretion may grant Performance Awards that are intended to qualify for the performance-based compensation exception under Section 162(m) and Performance Awards that are not intended so to qualify.

“Performance Criteria”:  Specified criteria, other than the mere continuation of Employment or the mere passage of time, the satisfaction of which is a condition for the grant, exercisability, vesting or full enjoyment of an Award.  For purposes of Awards that are intended to qualify for the performance-based compensation exception under Section 162(m), a Performance Criterion will mean an objectively determinable measure or measures of performance relating to any or any combination of the following (measured either absolutely or by reference to an index or indices and determined either on a consolidated basis or, as the context permits, on a divisional, subsidiary, line of business, project or geographical basis or in combinations thereof) : sales; revenues; assets; expenses; earnings before or after deduction for all or any portion of interest, taxes, depreciation, amortization or equity expense, whether or not on a continuing operations or an aggregate or per share basis; return on equity, investment, capital, capital employed or assets; one or more operating ratios; operating income or profit, including on an after-tax basis; net income; borrowing levels, leverage ratios or credit rating; market share; capital expenditures; cash flow; stock price; stockholder return; sales of particular products or services; customer acquisition or retention; acquisitions and divestitures (in whole or in part); joint ventures, strategic alliances, licenses or collaborations; spin-offs, split-ups and the like; reorganizations; recapitalizations, restructurings, financings (issuance of debt or equity) or refinancings;  manufacturing or process development; or achievement of clinical trial or research objectives, regulatory or other filings or approvals or other product development milestones.  A Performance Criterion and any targets with respect thereto determined by the Administrator need not be based upon an increase, a positive or improved result or avoidance of loss.  To the extent consistent with the requirements for satisfying the performance-based compensation exception under Section 162(m), the Administrator may provide in the case of any Award intended to qualify for such exception that one or more of the Performance Criteria applicable to such Award will be adjusted in an objectively determinable manner to reflect events (for example, the impact of charges for restructurings, discontinued operations, mergers, acquisitions, extraordinary items, and other unusual or non-recurring items, and the cumulative effects of tax or accounting changes, each as defined by U.S. generally accepted accounting principles) occurring during the performance period that affect the applicable Performance Criterion or Criteria.

13

“Plan”:  The Acceleron Pharma Inc. 2013 Equity Incentive Plan as from time to time amended and in effect.

“Restricted Stock”:  Stock subject to restrictions requiring that it be redelivered or offered for sale to the Company if specified conditions are not satisfied.

“Restricted Stock Unit”:  A Stock Unit that is, or as to which the delivery of Stock or cash in lieu of Stock is, subject to the satisfaction of specified performance or other vesting conditions.

“SAR”:  A right entitling the holder upon exercise to receive an amount (payable in cash or in shares of Stock of equivalent value) equal to the excess of the fair market value of the shares of Stock subject to the right over the base value from which appreciation under the SAR is to be measured.

“Section 409A”:  Section 409A of the Code.

“Section 422”:  Section 422 of the Code.

“Section 162(m)”:  Section 162(m) of the Code.

“Stock”:  Common stock of the Company, par value $0.001 per share.

“Stock Option”:  An option entitling the holder to acquire shares of Stock upon payment of the exercise price.

“Stock Unit”:  An unfunded and unsecured promise, denominated in shares of Stock, to deliver Stock or cash measured by the value of Stock in the future.

“Unrestricted Stock”:  Stock not subject to any restrictions under the terms of the Award.

14

EX-5.1

Exhibit 5.1

	ROPES & GRAY LLP
	PRUDENTIAL TOWER
	800 BOYLSTON STREET
	BOSTON, MA 02199-3600
	WWW.ROPESGRAY.COM

December 12, 2013

Acceleron Pharma Inc.

128 Sidney Street

Cambridge, MA  02139

Re:Registration Statement on Form S-8

Ladies and Gentlemen:

This opinion is furnished to you in connection with the registration statement on Form S-8 (the “Registration Statement”), filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), for the registration of an aggregate of 5,145,255 shares of common stock, $0.001 par value (the “Shares”), of Acceleron Pharma Inc., a Delaware corporation (the “Company”), issuable under the Company’s 2003 Stock Option and Restricted Stock Plan and the Company’s 2013 Equity Incentive Plan (each, a “Plan,” and collectively, the “Plans”).

We are familiar with the actions taken by the Company in connection with the adoption of the Plans.  For purposes of our opinion, we have examined and relied upon such documents, records, certificates and other instruments as we have deemed necessary.

The opinions expressed below are limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, when the Shares have been issued and sold in accordance with the applicable Plan, the Shares will be validly issued, fully paid and nonassessable.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement.  In giving this consent we do not thereby admit that we are included in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

Very truly yours,

/s/ Ropes & Gray LLP

EX-23.2

Exhibit 23.2

Consent of Independent Registered Public Accounting Firm

We consent to the incorporation by reference in the Registration Statement (Form S-8) pertaining to the Acceleron Pharma Inc. 2003 Stock Option and Restricted Stock Plan and the Acceleron Pharma Inc. 2013 Equity Incentive Plan of our report dated July 3, 2013, except for Note 16, as to which the date is September 5, 2013, with respect to the financial statements of Acceleron Pharma Inc. included in the Registration Statement on Form S-1 as amended (File No. 333-190417) and related Prospectus filed with the Securities and Exchange Commission.

/s/ Ernst & Young LLP

Boston, Massachusetts

December 11, 2013