Exhibit 10.1

FORM OF INDEMNIFICATION AGREEMENT

THIS INDEMNIFICATION AGREEMENT (this “Agreement”) is made as of [ ], 2013, by and between Acceleron Pharma Inc., a Delaware corporation (the “Company”) and [ ] (“Indemnitee”).

RECITALS

WHEREAS, although the Certificate of Incorporation and the Bylaws of the Company provide for indemnification of the officers and directors of the Company and Indemnitee may also be entitled to indemnification pursuant to the General Corporation Law of the State of Delaware (“DGCL”), the DGCL expressly contemplates that contracts may be entered into between the Company and its directors and officers with respect to indemnification of such directors and officers;

WHEREAS, Indemnitee’s continued service to the Company substantially benefits the Company;

WHEREAS, the Board of Directors of the Company (the “Board”) has determined that it is in the best interest of the Company and that it is reasonably prudent and necessary for the Company to contractually obligate itself to indemnify, and to advance expenses on behalf of, Indemnitee to the fullest extent permitted by applicable law in order to induce Indemnitee to serve or continue to serve the Company free from undue concern that Indemnitee will not be so indemnified or that any indemnification obligation will not be met;

WHEREAS, this Agreement is a supplement to and in furtherance of (a) the Certificate of Incorporation and Bylaws of the Company and (b) the certificate of incorporation, bylaws, partnership agreement or other organizational document, as the case may be, of any Enterprise (as defined below) and (c) any resolutions adopted pursuant thereto, and shall not be deemed a substitute therefor, nor to diminish or abrogate any rights of Indemnitee thereunder;

WHEREAS, Indemnitee does not regard the protection available under the Company’s Certificate of Incorporation, Bylaws and insurance or any other Enterprise’s certificate of incorporation, bylaws, partnership agreement or other organizational document, as the case may be, and insurance, as adequate in the present circumstances, and may not be willing to serve as an Agent (as defined below) without adequate protection, and the Company desires Indemnitee to serve in such capacity. Indemnitee is willing to serve, continue to serve and to take on additional service for or on behalf of the Company and certain other Enterprises on the condition that Indemnitee be so indemnified; and

[WHEREAS, Indemnitee may have certain rights to indemnification and/or insurance provided by other entities and/or organizations which Indemnitee and such other entities and/or organizations intend to be secondary to the primary obligation of the Company to indemnify

Indemnitee as provided herein, with the Company’s acknowledgement and agreement to the foregoing being a material condition to Indemnitee’s willingness to serve on the Board.](1)

NOW, THEREFORE, in consideration of the promises and the covenants contained herein, the Company and Indemnitee do hereby covenant and agree as follows:

AGREEMENT

1. Services to the Company and Certain Other Enterprises. Indemnitee will serve or continue to serve as an Agent for so long as Indemnitee is duly elected or appointed or until Indemnitee tenders a resignation from such position(s).

2. Definitions. As used in this Agreement:

(a) “Agent” means any person who is or was a director, officer, employee, trustee, general partner, managing member, fiduciary or other agent of (i) the Company, (ii) any Enterprise which is an affiliate or wholly or partially owned subsidiary of the Company; (iii) another Enterprise, at the request of, for the convenience of, or to represent the interests of the Company or any Enterprise which is an affiliate or wholly or partially owned subsidiary of the Company; (iv) an Enterprise which was a predecessor of the Company or a subsidiary of the Company, or (v) an Enterprise at the request of, for the convenience of, or to represent the interests of such predecessor of the Company or such other Enterprise.

(b) “Change of Control” means

(1) any “person” (as such term is used in Sections 13(d) and 14(d) of the Exchange Act), becomes the “Beneficial Owner” (as defined in Rule 13d-3 under the Exchange Act), directly or indirectly, of securities of the Company representing more than fifty percent (50%) of the total voting power represented by the Company’s then outstanding voting securities (excluding for this purpose any such voting securities held by the Company, or any parent or subsidiary of the Company or any employee benefit plan of the Company) pursuant to a transaction or a series of transactions which the Board does not approve;

(2) a merger or consolidation of the Company, whether or not approved by the Board, which results in the holders of voting securities of the Company outstanding immediately prior thereto failing to continue to represent (either by remaining outstanding or by being converted into voting securities of the surviving entity) at least fifty percent (50%) of the combined voting power of the voting securities of the Company or such surviving entity outstanding immediately after such merger or consolidation;

(1) To be included if Indemnitee is affiliated with a fund or other entity that provides indemnification to Indemnitee.

(3) the sale or disposition of all or substantially all of the Company’s assets (or consummation of any transaction having similar effect) provided that the sale or disposition is of more than two-thirds (2/3) of the assets of the Company; or

(4) the date a majority of members of the Board is replaced during any twelve (12) month period by directors whose appointment or election is not endorsed by a majority of the members of the Board before the date of such appointment or election.

(5) In any case, a Change of Control under this Section 2(b) must also meet the requirements of a change in ownership or effective control, or a sale of a substantial portion of the Company’s assets in accordance with Section 409A(a)(2)(A)(v) of the Internal Revenue Code of 1986, as amended, and the applicable provisions of Treasury Regulation § 1.409A-3.

(c) “Corporate Status” describes the status of a person who is or was an Agent of the Company or of any other Enterprise, at the request of, for the convenience of, or to represent the interests of the Company or any Enterprise which is an affiliate or wholly or partially owned subsidiary of the Company.

(d) “Disinterested Director” means a director of the Company who is not and was not a party to the Proceeding in respect of which indemnification is sought by Indemnitee.

(e) “Enterprise” means (i) the Company, and (ii) any other foreign or domestic corporation, partnership, limited liability company, joint venture, trust, employee benefit plan or other enterprise.

(f) “Exchange Act” means the Securities Exchange Act of 1934, as amended.

(g) “Expenses” includes all reasonable attorneys’ fees, retainers, court costs, transcript costs, fees of experts, witness fees, travel expenses, duplicating costs, printing and binding costs, telephone charges, postage, delivery service fees, and all other disbursements or expenses of the types customarily incurred in connection with prosecuting, defending, preparing to prosecute or defend, investigating, being or preparing to be a witness in, or otherwise participating in, a Proceeding. Expenses shall include such fees and expenses, and costs incurred in connection with any appeal resulting from any Proceeding, including without limitation the premium, security for, and other costs relating to any cost bond, supersedeas bond, or other appeal bond or its equivalent. Expenses, however, shall not include amounts paid in settlement by Indemnitee or the amount of judgments or fines against Indemnitee.

(h) “Independent Counsel” means, at any time, any law firm, or a member of a law firm, that (i) is experienced in matters of corporation law and (ii) is not, at such time, or has not been in the five years prior to such time, retained to represent: (1) the Company or Indemnitee in any matter material to either such party (other than with respect to matters concerning Indemnitee under this Agreement, or of other indemnities under similar

indemnification agreements), or (2) any other party to the Proceeding giving rise to a claim for indemnification hereunder. Notwithstanding the foregoing, the term “Independent Counsel” shall not include any person who, under the applicable standards of professional conduct then prevailing, would have a conflict of interest in representing either the Company or Indemnitee in an action to determine Indemnitee’s rights under this Agreement. The Company agrees to pay the reasonable fees and expenses of the Independent Counsel referred to above and to fully indemnify such counsel against any and all Expenses, claims, liabilities and damages arising out of or relating to this Agreement or its engagement pursuant hereto and to be jointly and severally liable therefor.

(i) “Proceeding” includes any threatened, pending or completed, formal or informal, action, suit, arbitration, mediation, alternate dispute resolution mechanism, investigation, inquiry, administrative hearing or any other actual, threatened or completed proceeding, whether brought by or in the right of the Company or its Board of Directors or a governmental authority or other party or otherwise and whether of a civil, criminal, administrative or investigative nature, including without limitation any such proceeding pending as of the date of this Agreement, in which Indemnitee was, is or will be involved as a party or otherwise by reason of Indemnitee’s Corporate Status, in each case whether or not serving in such capacity at the time any Expense, judgment, fine or amount paid in settlement is incurred for which indemnification, reimbursement, or advancement of Expenses can be provided under this Agreement.

3. Indemnity in Third-Party Proceedings. The Company shall be liable to indemnify Indemnitee in accordance with the provisions of this Section 3 if Indemnitee is, or is threatened to be made, a party to or a participant (as a witness or otherwise) in any Proceeding, other than a Proceeding by or in the right of the Company to procure a judgment in its favor. Pursuant to this Section 3, Indemnitee shall be indemnified against all Expenses, judgments, fines and amounts paid in settlement actually and reasonably incurred by Indemnitee or on Indemnitee’s behalf in connection with such Proceeding or any claim, issue or matter therein, if Indemnitee acted in good faith and in a manner Indemnitee reasonably believed to be in or not opposed to the best interests of the Company and, in the case of a criminal Proceeding, had no reasonable cause to believe that Indemnitee’s conduct was unlawful.

4. Indemnity in Proceedings by or in the Right of the Company. The Company shall be liable to indemnify Indemnitee in accordance with the provisions of this Section 4 if Indemnitee is, or is threatened to be made, a party to or a participant (as a witness or otherwise) in any Proceeding by or in the right of the Company to procure a judgment in its favor. Pursuant to this Section 4, Indemnitee shall be indemnified against all Expenses actually and reasonably incurred by Indemnitee or on Indemnitee’s behalf in connection with such Proceeding (or any claim, issue or matter therein) if Indemnitee acted in good faith and in a manner Indemnitee reasonably believed to be in or not opposed to the best interests of the Company; provided, however that no indemnification for Expenses shall be made under this Section 4 in respect of any claim, issue or matter as to which Indemnitee shall have been finally adjudged by a court of competent jurisdiction to be liable to the Company, unless and only to the extent that any court in which the Proceeding was brought shall determine that such indemnification may be made.

5. Indemnification for Expenses of a Party Who is Wholly or Partly Successful. Notwithstanding any other provisions of this Agreement, to the extent that Indemnitee is a party to (or a participant in) and is successful, on the merits or otherwise, in any Proceeding, the Company shall be liable to indemnify Indemnitee against all Expenses actually and reasonably incurred by him in connection therewith. If Indemnitee is not wholly successful in such Proceeding but is successful, on the merits or otherwise, as to one or more but less than all claims, issues or matters in such Proceeding, the Company shall be liable to indemnify Indemnitee against all Expenses actually and reasonably incurred by Indemnitee or on Indemnitee’s behalf in connection with each successfully resolved claim, issue or matter. For purposes of this Section and without limitation, the termination of any claim, issue or matter in such a Proceeding by dismissal, with or without prejudice, shall be deemed to be a successful result as to such claim, issue or matter.

6. Actions where Indemnitee is Deceased. If Indemnitee is a person who was or is a party or is threatened to be made a party to any Proceeding by reason of Indemnitee’s Corporate Status, or by reason of anything done or not done by Indemnitee in any such capacity, and if, prior to, during the pendency of or after completion of such Proceeding Indemnitee is deceased, the Company shall indemnify Indemnitee’s heirs, executors and administrators against all Expenses and liabilities of any type whatsoever to the extent Indemnitee would have been entitled to indemnification pursuant to this Agreement were Indemnitee still alive.

7. Indemnification For Expenses of a Witness. Notwithstanding any other provision of this Agreement, to the extent that Indemnitee is, by reason of Indemnitee’s Corporate Status, a witness in any Proceeding or is requested or compelled to respond to discovery requests in any Proceeding to which Indemnitee is not a party, the Company shall be liable to indemnify Indemnitee against all Expenses actually and reasonably incurred by Indemnitee or on Indemnitee’s behalf in connection therewith.

8. Additional Indemnification

(a) Notwithstanding any limitation in Sections 3, 4, or 5, the Company shall be liable to indemnify Indemnitee to the fullest extent permitted by law if Indemnitee is a party to, or threatened to be made a party to, any Proceeding (including a Proceeding by or in the right of the Company to procure a judgment in its favor) against all Expenses, judgments, fines and amounts paid in settlement actually and reasonably incurred by Indemnitee in connection with the Proceeding; provided, however, that no indemnity shall be made under this Section 8(a) on account of Indemnitee’s conduct which has been adjudicated to constitute a breach of Indemnitee’s duty of loyalty to the Company or its shareholders or to constitute an act or omission not in good faith or involving intentional misconduct or a knowing violation of the law.

(b) For purposes of Section 8(a), the meaning of the phrase “to the fullest extent permitted by law” shall include, but not be limited to:

(1) to the fullest extent permitted by the provision of the DGCL that authorizes or contemplates additional indemnification by agreement, or the corresponding provision of any amendment to or replacement of the DGCL; and

(2) to the fullest extent authorized or permitted by any amendments to or replacements of the DGCL adopted after the date of this Agreement that increase the extent to which a corporation may indemnify its officers and directors.

9. Exclusions. Notwithstanding any provision in this Agreement, the Company shall not be obligated under this Agreement to make any indemnity payment, or to advance any expenses, in connection with any claim made against Indemnitee:

(a) for which payment has actually been received by or on behalf of Indemnitee under any insurance policy or other indemnity provision [(other than with respect to any insurance policy or other indemnity provision which is provided to Indemnitee at the cost or expense of any Fund Indemnitor)](1), except with respect to any excess beyond the amount actually received under any insurance policy or other indemnity provision [or as provided in Section 15(c) below](1);

(b) for an accounting of profits made from the purchase and sale (or sale and purchase) by Indemnitee of securities of the Company within the meaning of Section 17(b) of the Exchange Act or similar provisions of state statutory law or common law; provided, however, that notwithstanding any limitation on the Company’s obligation to provide indemnification set forth in this Section 9(b) or elsewhere, Indemnitee shall be entitled to receive advancement of Expenses hereunder with respect to any such claim unless and until a court having jurisdiction over the claim shall have made a final judicial determination (as to which all rights of appeal therefrom have been exhausted or lapsed) that Indemnitee has violated said statute; or

(c) in connection with any Proceeding (or any part of any Proceeding) initiated by Indemnitee, including any Proceeding (or any part of any Proceeding) initiated by Indemnitee against the Company or its directors, officers, employees or other indemnitees, unless (i) such indemnification is expressly required to be made by applicable law, (ii) the Proceeding was authorized by the Board, (iii) such indemnification is provided by the Company, in its sole discretion, pursuant to the powers vested in the Company under the DGCL, or (iv) such indemnification is required to be made pursuant to Section 14 of this Agreement.

10. Advancement of Expenses; Defense of Claim. Except as otherwise provided herein, the Company shall be obligated to advance any and all Expenses incurred by Indemnitee in connection with any Proceeding within ten (10) days after the receipt by the Company of a statement or statements requesting such advances from time to time, whether prior to or after final disposition of any Proceeding. Such statement or statements shall reasonably evidence the Expenses incurred by Indemnitee and shall include or be preceded or accompanied by a written undertaking by or on behalf of Indemnitee to repay any Expenses advanced to the extent and only to the extent that it is ultimately determined that Indemnitee is not entitled to be indemnified by the Company. Any advances (i) shall be unsecured and interest free; (ii) shall be made

without regard to Indemnitee’s ability to repay the advances and without regard to Indemnitee’s ultimate entitlement to indemnification under the other provisions of this Agreement; and (iii) shall include any and all reasonable Expenses incurred pursuing an action to enforce this right of advancement, including Expenses incurred preparing and forwarding statements to the Company to support the advances claimed. The Company will be entitled to participate reasonably in the Proceeding at its own expense.

11. Procedure for Notification and Requests for Advancement and Indemnification.

(a) Notification. To obtain advancement of Expenses and/or indemnification under this Agreement, Indemnitee shall, not later than sixty (60) days after receipt by Indemnitee of notice of the commencement of any Proceeding, except for Proceedings pending as of the date of this Agreement, submit to the Company written notification of the Proceeding; with regard to Proceedings pending as of the date of this Agreement, Indemnitee shall submit to the Company written notification not later than thirty (30) days after the date of this Agreement. The omission to notify the Company will relieve the Company of its advancement or indemnification obligations under this Agreement only to the extent the Company can establish that such omission to notify resulted in actual prejudice to it, and the omission to notify the Company will, in any event, not relieve the Company from any liability which it may have to indemnify Indemnitee otherwise than under this Agreement. The Secretary of the Company shall, promptly upon receipt of notification from Indemnitee pursuant to this Section 11(a), advise the Board in writing that Indemnitee has provided such notification.

(b) Expense Request. Subject to Section 10, to obtain advancement of Expenses under this Agreement, Indemnitee shall submit to the Company a written request therefor, together with such invoices or other supporting information as may be reasonably requested by the Company and reasonably available to Indemnitee, and, only to the extent required by applicable law which cannot be waived, an unsecured written undertaking to repay amounts advanced. The Company shall make advance payment of Expenses to Indemnitee no later than ten (10) days after receipt of the written request for advancement (and each subsequent request for advancement) by Indemnitee. If, at the time of receipt of any such written request for advancement of Expenses, the Company has director and officer insurance policies in effect, the Company will promptly notify the relevant insurers in accordance with the procedures and requirements of such policies. The Company shall thereafter keep such director and officer insurers informed of the status of the Proceeding or other claim, as appropriate to secure insurance coverage of Indemnitee for such claim.

(c) Indemnification Request. In order to obtain indemnification under this Agreement, Indemnitee shall, anytime at Indemnitee’s discretion following notification by Indemnitee of the commencement of any Proceeding pursuant to Section 11(a) of this Agreement and consistent with the time period for the duration of this Agreement as set forth in Section 17 of this Agreement, submit to the Company a written request for indemnification pursuant to this Section 11(c), including therein or therewith such documentation and information as is reasonably available to Indemnitee and is reasonably necessary to determine whether and to what extent Indemnitee is entitled to indemnification. No determination of Indemnitee’s entitlement

to indemnification shall be made until such written request for a determination is submitted by Indemnitee to the Company pursuant to this Section 11(c). The failure to submit a written request to the Company will relieve the Company of its indemnification obligations under this Agreement only to the extent the Company can establish that such failure to make a written request resulted in actual prejudice to it, and the failure to make a written request will not relieve the Company from any liability which it may have to indemnify Indemnitee otherwise than under this Agreement. The Secretary of the Company shall, promptly upon receipt of such a request for indemnification, advise the Board in writing that Indemnitee has requested indemnification. Upon submission of a written request for indemnification by Indemnitee pursuant to this Section 11(c), Indemnitee’s entitlement to indemnification shall be determined according to Section 12 of this Agreement.

12. Procedure Upon Application for Indemnification.

(a) Upon receipt of Indemnitee’s written request for indemnification pursuant to Section 11(c), a determination with respect thereto shall be made in the specific case by one of the following four methods, which shall be at the election of the Board: (i) by a majority vote of the Disinterested Directors, even though less than a quorum, (ii) by a committee of Disinterested Directors designated by a majority vote of the Disinterested Directors, even though less than a quorum, (iii) if there are no Disinterested Directors or if the Disinterested Directors so direct, by Independent Counsel in a written opinion to the Board, a copy of which shall be delivered to Indemnitee or (iv) by the stockholders of the Company. Notwithstanding the above, if a determination with respect to Indemnitee’s right to indemnification is to be made following a Change of Control, such determination shall be made in the specific case by Independent Counsel in a written opinion to the Board, a copy of which shall be delivered to Indemnitee. If it is so determined that Indemnitee is entitled to indemnification, payment to Indemnitee shall be made within ten (10) days after such determination. Indemnitee shall reasonably cooperate with the person, persons or entity making such determination with respect to Indemnitee’s entitlement to indemnification, including providing to such person, persons or entity upon reasonable advance request any documentation or information which is not privileged or otherwise protected from disclosure and which is reasonably available to Indemnitee and reasonably necessary to such determination. Any Expenses incurred by Indemnitee in so cooperating with the Disinterested Directors or Independent Counsel, as the case may be, making such determination shall be advanced and borne by the Company (irrespective of the determination as to Indemnitee’s entitlement to indemnification) and the Company is liable to indemnify and hold Indemnitee harmless therefrom.

(b) In the event the determination of entitlement to indemnification is to be made by Independent Counsel pursuant to Section 12(a) hereof, the Independent Counsel shall be selected as provided in this Section 12(b). The Independent Counsel shall be selected by the Board and the Board shall provide written notice to Indemnitee of the identity of the Independent Counsel so selected. Indemnitee may, within ten (10) days after such written notice of selection shall have been received, deliver to the Company a written objection to such selection; provided, however, that such objection may be asserted only on the ground that the Independent Counsel so selected does not meet the requirements of “Independent Counsel” as defined in Section 2 of

this Agreement, and the objection shall set forth with particularity the factual basis of such assertion. Absent a proper and timely objection, the person so selected shall act as Independent Counsel. If a written objection is so made and substantiated, the Independent Counsel so selected may not serve as Independent Counsel unless and until such objection is withdrawn or a court of competent jurisdiction has determined that such objection is without merit. If, within twenty (20) days after submission by Indemnitee of a written request for indemnification pursuant to Section 11(c) hereof, no Independent Counsel shall have been selected and not objected to, either the Company or Indemnitee may petition a court of competent jurisdiction for resolution of any objection which shall have been made by Indemnitee to the Company’s selection of Independent Counsel and/or for the appointment as Independent Counsel of a person selected by the Court or by such other person as the Court shall designate, and the person with respect to whom all objections are so resolved or the person so appointed shall act as Independent Counsel under Section 12(a) hereof. Upon the due commencement of any judicial proceeding or arbitration pursuant to Section 14(a) of this Agreement, Independent Counsel shall be discharged and relieved of any further responsibility in such capacity (subject to the applicable standards of professional conduct then prevailing). The Company shall pay all reasonable fees and expenses incident to the procedures of this Section 12(b), regardless of the manner in which such Independent Counsel was selected or appointed.

13. Presumptions and Effect of Certain Proceedings.

(a) In making a determination with respect to entitlement to indemnification hereunder, the person or persons or entity making such determination shall presume that Indemnitee is entitled to indemnification under this Agreement if Indemnitee has submitted a notice and a request for indemnification in accordance with Section 11 of this Agreement. Anyone seeking to overcome this presumption shall have the burden of proof and the burden of persuasion by clear and convincing evidence. Neither the failure of the Company (including by the Board) or of Independent Counsel to have made a determination prior to the commencement of any judicial proceeding or arbitration pursuant to this Agreement that indemnification is proper in the circumstances because Indemnitee has met the applicable standard of conduct, nor an actual determination by the Company (including by the Board) or by Independent Counsel that Indemnitee has not met such applicable standard of conduct, shall be a defense to the action or create a presumption that Indemnitee has not met the applicable standard of conduct.

(b) If the person, persons or entity empowered or selected under Section 12 of this Agreement to determine whether Indemnitee is entitled to indemnification shall not have made a determination within sixty (60) days after receipt by the Company of Indemnitee’s written request for indemnification pursuant to Section 11(c) of this Agreement, the requisite determination of entitlement to indemnification shall be deemed to have been made and Indemnitee shall be entitled to such indemnification, absent (i) a misstatement by Indemnitee of a material fact, or an omission of a material fact necessary to make Indemnitee’s statement not materially misleading, in connection with the request for indemnification, or (ii) a prohibition of such indemnification under applicable law; provided, however, that such 60-day period may be extended for a reasonable time, not to exceed an additional thirty (30) days, if the person, persons or entity making the determination with respect to entitlement to indemnification in good

faith requires such additional time for the obtaining or evaluating of documentation and/or information relating thereto.

(c) The termination of any Proceeding or of any claim, issue or matter therein, by judgment, order, settlement or conviction, or upon a plea of nolo contendere or its equivalent, shall not (except as otherwise expressly provided in this Agreement) of itself adversely affect the right of Indemnitee to indemnification or create a presumption that Indemnitee did not act in good faith and in a manner which Indemnitee reasonably believed to be in or not opposed to the best interests of the Company or, with respect to any criminal Proceeding, that Indemnitee had reasonable cause to believe that Indemnitee’s conduct was unlawful.

(d) Reliance as Safe Harbor. For purposes of any determination of good faith, Indemnitee shall be deemed to have acted in good faith if Indemnitee’s action or failure to act is based on the records or books of account of the applicable Enterprise relating to the Indemnitee’s Corporate Status, including financial statements, or on information supplied to Indemnitee by the officers of such Enterprise in the course of their duties, or on the advice of legal counsel for such Enterprise or on information or records given or reports made to such Enterprise by an independent certified public accountant or by an appraiser or other expert selected by such Enterprise. The provisions of this Section 13(d) shall not be deemed to be exclusive or to limit in any way the other circumstances in which Indemnitee may be deemed or found to have met the applicable standard of conduct set forth in this Agreement.

(e) Actions of Others. The knowledge and/or actions, or failure to act, of any other Agent of the Company or any other Enterprise relating to the Indemnitee’s Corporate Status shall not be imputed to Indemnitee for purposes of determining Indemnitee’s right to indemnification under this Agreement.

14. Remedies of Indemnitee.

(a) In the event that (i) a determination is made pursuant to Section 12 of this Agreement that Indemnitee is not entitled to indemnification under this Agreement, (ii) advancement of Expenses is not timely made pursuant to Section 10 or 11(b) of this Agreement, (iii) payment of indemnification is not made pursuant to Section 5, 6, 7 or the last sentence of Section 12(a) of this Agreement within ten (10) days after receipt by the Company of a written request therefor, or (iv) payment of indemnification pursuant to Section 3 or 4 of this Agreement is not made within ten (10) days after a determination has been made that Indemnitee is entitled to indemnification, or (v) Indemnitee determines in its sole discretion that such action is appropriate or desirable, Indemnitee shall be entitled to seek an adjudication by a court of competent jurisdiction as to Indemnitee’s entitlement to such indemnification or advancement of Expenses. Alternatively, Indemnitee, at Indemnitee’s option, may seek an award in arbitration to be conducted by a single arbitrator pursuant to the Commercial Arbitration Rules of the American Arbitration Association. The Company shall not oppose Indemnitee’s right to seek any such adjudication or award in arbitration.

(b) In the event that a determination shall have been made pursuant to Section 12(a) of this Agreement that Indemnitee is not entitled to indemnification, any judicial proceeding or arbitration, commenced pursuant to this Section 14, shall be conducted in all respects as a de novo trial, or arbitration, on the merits, and Indemnitee shall not be prejudiced by reason of that adverse determination. In any judicial proceeding or arbitration commenced pursuant to this Section 14, in the event that the person, persons or entity empowered or selected under Section 12 of this Agreement to determine whether Indemnitee is entitled to indemnification has not made such a determination within the time period provided for under Section 13(b) of this Agreement, the Company shall stipulate and may not contest that Indemnitee acted in good faith and in a manner Indemnitee reasonably believed to be in or not opposed to the best interests of the Company, and, with respect to any criminal action or Proceeding, had no reasonable cause to believe Indemnitee’s conduct was unlawful.

(c) If a determination shall have been made pursuant to Section 12(a) of this Agreement that Indemnitee is entitled to indemnification, the Company shall be bound by such determination in any judicial proceeding or arbitration commenced pursuant to this Section 14, absent (i) a misstatement by Indemnitee of a material fact, or an omission of a material fact necessary to make Indemnitee’s statement not materially misleading, in connection with the request for indemnification, or (ii) a prohibition of such indemnification under applicable law.

(d) In the event that Indemnitee is a party to a judicial proceeding or arbitration pursuant to this Section 14 concerning Indemnitee’s rights under, or to recover damages for breach of, this Agreement, Indemnitee shall be entitled to recover from the Company, and shall be indemnified by the Company against, any and all Expenses actually and reasonably incurred by him in such judicial adjudication or arbitration. If it shall be determined in said judicial adjudication or arbitration that Indemnitee is entitled to receive part but not all of the indemnification or advancement of Expenses sought, Indemnitee shall be entitled to recover from the Company (who shall be liable therefor), and shall be indemnified by the Company against, any and all Expenses reasonably incurred by Indemnitee in connection with such judicial adjudication or arbitration.

(e) The Company shall be precluded from asserting in any judicial proceeding or arbitration commenced pursuant to this Section 14 that the procedures and presumptions of this Agreement are not valid, binding and enforceable and shall stipulate in any such court or before any such arbitrator that the Company is bound by all the provisions of this Agreement. The Company shall be liable to indemnify Indemnitee against any and all Expenses and, if requested by Indemnitee, shall (within ten (10) days after receipt by the Company of a written request therefor) advance such Expenses to Indemnitee that are incurred by Indemnitee in connection with any judicial adjudication or arbitration involving Indemnitee for indemnification or advancement of Expenses from the Company under this Agreement or under any directors’ and officers’ liability insurance policies maintained by the Company, regardless of whether Indemnitee ultimately is determined to be entitled to such indemnification, advancement of Expenses or insurance recovery, as the case may be.

15. Non-Exclusivity; Survival of Rights; Insurance.

(a) The rights of indemnification and to receive advancement of Expenses as provided by this Agreement shall not be deemed exclusive of any other rights to which Indemnitee may at any time be entitled under applicable law, the Company’s or any other Enterprise’s Certificate of Incorporation, Bylaws or similar organizational documents, any agreement, a vote of stockholders or a resolution of directors, or otherwise. No amendment, alteration or repeal of this Agreement or of any provision hereof shall limit or restrict any right of Indemnitee under this Agreement in respect to any action taken or omitted by such Indemnitee in Indemnitee’s Corporate Status prior to such amendment, alteration or repeal. To the extent that a change in Delaware law, whether by statute or judicial decision, permits greater indemnification or advancement of Expenses than would be afforded currently under the Company’s or any other Enterprise’s Certificate of Incorporation, Bylaws and this Agreement, it is the intent of the parties hereto that Indemnitee shall enjoy by this Agreement the greater benefits so afforded by such change. No right or remedy herein conferred is intended to be exclusive of any other right or remedy, and every other right and remedy shall be cumulative and in addition to every other right and remedy given hereunder or now or hereafter existing at law or in equity or otherwise. The assertion or employment of any right or remedy hereunder, or otherwise, shall not prevent the concurrent assertion or employment of any other right or remedy.

(b) To the extent that the Company maintains an insurance policy or policies providing liability insurance for Agents of the Company or of any other Enterprise, Indemnitee shall be covered by such policy or policies in accordance with its or their terms to the maximum extent of the coverage available for any such Agent under such policy or policies. If, at the time of the receipt of a notice of a claim pursuant to Section 11(a) hereof, the Company has director and officer liability insurance in effect, the Company shall give prompt notice of the commencement of such Proceeding to the insurers in accordance with the procedures set forth in the respective policies. The Company shall thereafter take all necessary or desirable action to cause such insurers to pay, on behalf of Indemnitee, all amounts payable as a result of such Proceeding in accordance with the terms of such policies.

(c) [The Company hereby acknowledges that Indemnitee has certain rights to indemnification, advancement of expenses and/or insurance provided by [Name of Fund/Sponsor] and certain of [its][their] affiliates (collectively, the “Fund Indemnitors”). The Company hereby agrees (i) that it is the indemnitor of first resort (i.e., its obligations to Indemnitee are primary and any obligation of the Fund Indemnitors to advance expenses or to provide indemnification for the same expenses or liabilities incurred by Indemnitee are secondary), (ii) that it shall be required to advance the full amount of expenses incurred by Indemnitee and shall be liable for the full amount of all Expenses, judgments, penalties, fines and amounts paid in settlement to the extent legally permitted and as required by the terms of this Agreement and the Certificate of Incorporation or Bylaws of the Company (or any other agreement between the Company and Indemnitee), without regard to any rights Indemnitee may have against the Fund Indemnitors, and, (iii) that it irrevocably waives, relinquishes and releases the Fund Indemnitors from any and all claims against the Fund Indemnitors for contribution, subrogation or any other recovery of any kind in respect thereof. The Company further agrees that no advancement or payment by the Fund Indemnitors on behalf of Indemnitee with respect to any claim for which Indemnitee has sought indemnification from the Company shall affect the

foregoing and the Fund Indemnitors shall have a right of contribution and/or be subrogated to the extent of such advancement or payment to all of the rights of recovery of Indemnitee against the Company. The Company and Indemnitee agree that the Fund Indemnitors are express third party beneficiaries of the terms of this Section 15(c).

(d) Except as provided in paragraph (c) above, in the event of any payment under this Agreement, the Company shall be subrogated to the extent of such payment to all of the rights of recovery of Indemnitee (other than against the Fund Indemnitors), who shall execute all papers required and take all action necessary to secure such rights, including execution of such documents as are necessary to enable the Company to bring suit to enforce such rights.](1)

(e) [Except as provided in paragraph (c) above,](1) the Company shall not be liable under this Agreement to make any payment of amounts otherwise indemnifiable (or for which advancement is provided hereunder) hereunder only to the extent that Indemnitee has otherwise actually received such payment under any insurance policy, contract, agreement or otherwise.

(f) [Except as provided in paragraph (c) above, ](1) the Company’s obligation hereunder to indemnify or advance Expenses to Indemnitee by reason of Indemnitee’s Corporate Status shall be reduced by any amount Indemnitee has actually received as indemnification or advancement of Expenses from such other Enterprise.

16. Liability Insurance.

(a) The Company hereby covenants and agrees that, so long as Indemnitee shall continue to serve as an Agent of the Company and thereafter so long as Indemnitee shall be subject to any possible Proceeding by reason of the fact of Indemnitee’s Corporate Status, the Company shall promptly obtain and maintain in full force and effect directors’ and officers’ liability insurance (“D&O Insurance”) in reasonable amounts from established and reputable insurers, as more fully described below. In the event of a Change in Control, the Company shall use reasonable best efforts to either: (i) maintain such D&O Insurance for six years; or (ii) purchase a six year tail for such D&O Insurance.

(b) In all policies of D&O Insurance, Indemnitee shall qualify as an insured in such a manner as to provide Indemnitee the same rights and benefits as are accorded to the most favorably insured of the Company’s independent directors (as defined by the insurer) if Indemnitee is such an independent director; of the Company’s non-independent directors if Indemnitee is not an independent director; of the Company’s officers if Indemnitee is an officer of the Company; or of the Company’s key employees, if Indemnitee is not a director or officer but is a key employee.

17. Duration of Agreement. All agreements and obligations of the Company contained herein shall continue during the period Indemnitee is an Agent of the Company (or is or was serving at the request of the Company as an Agent of any other Enterprise and shall continue thereafter so long as Indemnitee shall be subject to any Proceeding (or any proceeding commenced under Section 14 or Section 15 hereof) by reason of his or her Corporate Status, whether or not he or she is acting or serving in any capacity at the time any liability or Expense is incurred for which indemnification can be provided under this Agreement. This Agreement shall be binding upon and inure to the benefit of and be enforceable by the parties hereto and their respective successors (including any direct or indirect successor by purchase, merger, consolidation or otherwise to all or substantially all of the business or assets of the Company), assigns, spouses, heirs, executors and personal and legal representative.

18. Severability. If any provision or provisions of this Agreement shall be held to be invalid, illegal or unenforceable for any reason whatsoever: (a) the validity, legality and enforceability of the remaining provisions of this Agreement (including, without limitation, each portion of any Section of this Agreement containing any such provision held to be invalid, illegal or unenforceable, that is not itself invalid, illegal or unenforceable) shall not in any way be affected or impaired thereby and shall remain enforceable to the fullest extent permitted by law; (b) such provision or provisions shall be deemed reformed to the extent necessary to conform to applicable law and to give the maximum effect to the intent of the parties hereto; and (c) to the fullest extent possible, the provisions of this Agreement (including, without limitation, each portion of any section of this Agreement containing any such provision held to be invalid, illegal or unenforceable, that is not itself invalid, illegal or unenforceable) shall be construed so as to give effect to the intent manifested thereby.

19. Enforcement.

(a) The Company expressly confirms and agrees that it has entered into this Agreement and assumed the obligations imposed on it hereby in order to induce Indemnitee to continue to serve as an Agent of the Company, and the Company acknowledges that Indemnitee is relying upon this Agreement in serving as an Agent of the Company.

(b) This Agreement constitutes the entire agreement between the parties hereto with respect to the subject hereof and supersedes any and all prior agreements and understandings, oral, written and implied, between the parties hereto with respect to the subject matter hereof.

20. Modification and Waiver. No supplement, modification or amendment of this Agreement shall be binding unless executed in writing by the parties hereto. No waiver of any of the provisions of this Agreement shall be deemed or shall constitute a waiver of any other provisions of this Agreement nor shall any waiver constitute a continuing waiver.

21. Notice by Indemnitee. Indemnitee agrees promptly to notify the Company in writing upon being served with any summons, citation, subpoena, complaint, indictment, information or other document relating to any Proceeding or matter which may be subject to

indemnification or advancement of Expenses covered hereunder. The failure of Indemnitee to so notify the Company shall not relieve the Company of any obligation which it may have to Indemnitee under this Agreement or otherwise.

22. Notices. All notices, requests, demands and other communications under this Agreement shall be in writing and shall be deemed to have been duly given (a) if delivered by hand and receipted for by the party to whom said notice or other communication shall have been directed, (b) when sent by confirmed electronic mail or facsimile if sent during normal business hours of the recipient, and if not so confirmed, then on the next business day, (c) if mailed by certified or registered mail with postage prepaid, on the third business day after the date on which it is so mailed, or (d) one (1) day after deposit with a nationally recognized overnight courier, specifying next day delivery, with written verification:

(a) If to Indemnitee, at the address indicated on the signature page of this Agreement, or such other address as Indemnitee shall provide in writing to the Company,

(b) If to the Company to: Acceleron Pharma Inc.

128 Sidney Street

Cambridge, MA 02139

Attention: John Quisel, General Counsel

E-Mail: jquisel@acceleronpharma.com

or to any other address as may have been furnished to Indemnitee in writing by the Company.

23. Contribution. To the fullest extent permissible under applicable law, if the indemnification provided for in this Agreement is unavailable to Indemnitee for any reason whatsoever, the Company, in lieu of indemnifying Indemnitee, shall contribute to the amount incurred by Indemnitee, whether for judgments, fines, penalties, excise taxes, amounts paid or to be paid in settlement and/or for Expenses, in connection with any claim relating to an indemnifiable event under this Agreement, in such proportion in order to reflect (i) the relative benefits received by the Company and Indemnitee as a result of the event(s) and/or transaction(s) giving cause to such Proceeding; and/or (ii) the relative fault of the Company (and its directors, officer, employees and agents) and Indemnitee in connection with such event(s) and/or transaction(s).

24. Applicable Law and Consent to Jurisdiction. This Agreement and the legal relations among the parties shall be governed by, and construed and enforced in accordance with, the laws of the State of Delaware, without regard to its conflict of laws rules. Except with respect to any arbitration commenced by Indemnitee pursuant to Section 14(a) of this Agreement, the Company and Indemnitee hereby irrevocably and unconditionally (i) agree that any action or proceeding arising out of or in connection with this Agreement shall be brought only in the Chancery Court of the State of Delaware (the “Delaware Court”), and not in any other state or federal court in the United States of America or any court in any other country (ii) consent to submit to the exclusive jurisdiction of the Delaware Court for purposes of any action or proceeding arising out of or in connection with this Agreement, (iii) appoint, to the

extent such party is not a resident of the State of Delaware, irrevocably the Corporation Trust Company as its agent in the State of Delaware as such party’s agent for acceptance of legal process in connection with any such action or proceeding against such party with the same legal force and validity as if served upon such party personally within the State of Delaware, (iv) waive any objection to the laying of venue of any such action or proceeding in the Delaware Court, and (v) waive, and agree not to plead or to make, any claim that any such action or proceeding brought in the Delaware Court has been brought in an improper or inconvenient forum.

25. Counterparts. This Agreement may be executed in two or more counterparts, each of which shall for all purposes be deemed to be an original, but all of which together shall constitute one and the same Agreement. This Agreement may also be executed and delivered by facsimile signature and in two or more counterparts, each of which shall be deemed an original, but all of which together shall constitute one and the same instrument.

26. Headings. The headings of the paragraphs of this Agreement are inserted for convenience only and shall not be deemed to constitute part of this Agreement or to affect the construction thereof.

[Remainder of this page intentionally blank]

IN WITNESS WHEREOF, the parties have caused this Agreement to be signed as of the day and year first above written.

	ACCELERON PHARMA INC.


	By:
		Name:
		Title:


	INDEMNITEE


	Name:

	Address:




	E-mail: