XLRN Acceleron Pharma Inc

FORM 4 [X] Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).         
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES                                                                                   
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Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940                       

1. Name and Address of Reporting Person * Backstrom Jay T.	2. Issuer Name and Ticker or Trading Symbol ACCELERON PHARMA INC [ XLRN ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director                    _____ 10% Owner __X__ Officer (give title below)    _____ Other (specify below) EVP, Research and Development
(Last)          (First)          (Middle) 128 SIDNEY STREET	3. Date of Earliest Transaction (MM/DD/YYYY) 11/19/2021
(Street) CAMBRIDGE, MA 02139 (City)        (State)        (Zip)	4. If Amendment, Date Original Filed (MM/DD/YYYY)	6. Individual or Joint/Group Filing (Check Applicable Line) _X _ Form filed by One Reporting Person ___ Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security (Instr. 3)	2. Trans. Date	2A. Deemed Execution Date, if any	3. Trans. Code (Instr. 8)		4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	11/19/2021		U(1)		21233 (1)	D	(1)	0	D

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivate Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Trans. Date	3A. Deemed Execution Date, if any	4. Trans. Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Option to Purchase Common Stock	$52.61	11/19/2021		D			96963	(2)	(2)	Common Stock	96963	(2)	0	D
Option to Purchase Common Stock	$115.53	11/19/2021		D			20896	(2)	(2)	Common Stock	20896	(2)	0	D

Explanation of Responses:
(1)	This Form 4 reports securities disposed of pursuant to the terms of the Agreement and Plan of Merger (the "Merger Agreement"), dated as of September 29, 2021, by and among the Issuer, Merck Sharp & Dohme Corp. ("Parent"), and Astros Merger Sub, Inc., a direct wholly owned subsidiary of Parent ("Merger Sub"), pursuant to which Merger Sub completed a tender offer for shares of common stock of the Issuer (each, a "Share") and thereafter merged with and into the Issuer (the "Merger") effective as of November 19, 2021 (the "Effective Time"). At the Effective Time, each issued and outstanding Share was cancelled and converted into the right to receive $180.00 in cash (the "Offer Price") without interest and subject to applicable withholding taxes.
(2)	Pursuant to the terms of the Merger Agreement, at the Effective Time, each option to purchase Shares granted under an Issuer equity plan (each, a "Company Stock Option") that was outstanding and unvested immediately prior to the Effective Time vested in full. As of the Effective Time, each Company Stock Option that was outstanding immediately prior to the Effective Time was cancelled, and, in exchange therefor, the holder of such cancelled Company Stock Option was entitled to receive (without interest), in consideration of the cancellation of such Company Stock Option, an amount in cash (less applicable tax withholdings) equal to the product of (x) the total number of Shares subject to such Company Stock Option immediately prior to the Effective Time multiplied by (y) the excess, if any, of the Offer Price over the applicable exercise price per Share under such Company Stock Option.

Reporting Owners
Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Backstrom Jay T. 128 SIDNEY STREET CAMBRIDGE, MA 02139			EVP, Research and Development

Signatures
/s/ Adam M. Veness, as attorney-in-fact for Jay T. Backstrom		11/22/2021
**Signature of Reporting Person		Date