SEC FORM 3

SEC Form 3

FORM 3

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person^*

ORBIMED ADVISORS LLC

(Last)

(First)

(Middle)

C/O ORBIMED ADVISORS LLC

601 LEXINGTON AVENUE, 54TH FLOOR

(Street)

NEW YORK

10022

(City)

(State)

(Zip)

2. Date of Event Requiring Statement (Month/Day/Year)
09/18/2013

3. Issuer Name and Ticker or Trading Symbol
ACCELERON PHARMA INC [ XLRN ]

4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

Director	X	10% Owner
Officer (give title below)		Other (specify below)

5. If Amendment, Date of Original Filed (Month/Day/Year)

6. Individual or Joint/Group Filing (Check Applicable Line)

	Form filed by One Reporting Person
X	Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Beneficially Owned
1. Title of Security (Instr. 4)	2. Amount of Securities Beneficially Owned (Instr. 4)	3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	4. Nature of Indirect Beneficial Ownership (Instr. 5)
Common Stock	5,825	I	See footnote⁽¹⁾⁽²⁾⁽⁴⁾
Common Stock	2,181	I	See footnote⁽³⁾⁽⁴⁾

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
1. Title of Derivative Security (Instr. 4)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
Series B Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	1,127,346	⁽⁶⁾	I	See footnote⁽¹⁾⁽²⁾⁽⁴⁾
Series B Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	422,101	⁽⁶⁾	I	See footnote⁽³⁾⁽⁴⁾
Series C Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	175,419	⁽⁶⁾	I	See footnote⁽¹⁾⁽²⁾⁽⁴⁾
Series C Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	65,680	⁽⁶⁾	I	See footnote⁽³⁾⁽⁴⁾
Series D Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	23,442	⁽⁶⁾	I	See footnote⁽¹⁾⁽²⁾⁽⁴⁾
Series D Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	8,777	⁽⁶⁾	I	See footnote⁽³⁾⁽⁴⁾
Series E Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	169,803	⁽⁶⁾	I	See footnote⁽¹⁾⁽²⁾⁽⁴⁾
Series E Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	63,576	⁽⁶⁾	I	See footnote⁽³⁾⁽⁴⁾
Series F Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	38,924	⁽⁶⁾	I	See footnote⁽¹⁾⁽²⁾⁽⁴⁾
Series F Convertible Preferred Stock	⁽⁵⁾	⁽⁶⁾	Common Stock	14,574	⁽⁶⁾	I	See footnote⁽³⁾⁽⁴⁾
Warrants to Purchase Common Stock	⁽⁷⁾	⁽⁷⁾	Common Stock	88,749	$5.88	I	See footnote⁽¹⁾⁽²⁾⁽⁴⁾
Warrants to Purchase Common Stock	⁽⁷⁾	⁽⁷⁾	Common Stock	33,229	$5.88	I	See footnote⁽³⁾⁽⁴⁾
Warrants to Purchase Common Stock	⁽⁸⁾	⁽⁸⁾	Common Stock	24,151	$5.88	I	See footnote⁽¹⁾⁽²⁾⁽⁴⁾
Warrants to Purchase Common Stock	⁽⁸⁾	⁽⁸⁾	Common Stock	9,042	$5.88	I	See footnote⁽³⁾⁽⁴⁾

1. Name and Address of Reporting Person^*

ORBIMED ADVISORS LLC

(Last)

(First)

(Middle)

C/O ORBIMED ADVISORS LLC

601 LEXINGTON AVENUE, 54TH FLOOR

(Street)

NEW YORK

10022

(City)

(State)

(Zip)

Relationship of Reporting Person(s) to Issuer

Director	X	10% Owner
Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

ISALY SAMUEL D

(Last)

(First)

(Middle)

601 LEXINGTON AVENUE, 54TH FLOOR

(Street)

NEW YORK

10022

(City)

(State)

(Zip)

Relationship of Reporting Person(s) to Issuer

Director	X	10% Owner
Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

ORBIMED CAPITAL GP II LLC

(Last)

(First)

(Middle)

601 LEXINGTON AVENUE, 54TH FLOOR

(Street)

NEW YORK

10022

(City)

(State)

(Zip)

Relationship of Reporting Person(s) to Issuer

Director	X	10% Owner
Officer (give title below)		Other (specify below)

Explanation of Responses:

1. These shares of common stock, shares of Series B Convertible Preferred Stock, shares of Series C Convertible Preferred Stock, shares of Series D Convertible Preferred Stock, shares of Series E Convertible Preferred Stock, shares of Series F Convertible Preferred Stock and warrants to purchase common stock of the registrant are directly owned by OrbiMed Private Investments II, LP ("OPI II") and may be deemed to be indirectly beneficially owned by OrbiMed Capital GP II LLC ("GP II"), OrbiMed Advisors LLC ("Advisors") and Samuel D. Isaly ("Isaly"). GP II is the sole general partner of OPI II. Advisors, a registered adviser under the Investment Advisers Act of 1940, as amended, is the sole managing member of GP II. By virtue of such relationships, GP II and Advisors may be deemed to have voting and investment power over the securities held by OPI II noted above. Accordingly, GP II and Advisors may be deemed to have beneficial ownership of such securities.

2. Isaly, a natural person, may also be deemed to have voting and investment power over the securities held by OPI II noted above in his capacity as the managing member of, and owner of a controlling interest in, Advisors. Accordingly, Isaly may be deemed to have beneficial ownership of such securities.

3. These shares of common stock, shares of Series B Convertible Preferred Stock, shares of Series C Convertible Preferred Stock, shares of Series D Convertible Preferred Stock, shares of Series E Convertible Preferred Stock, shares of Series F Convertible Preferred Stock and warrants to purchase common stock of the registrant are directly owned by OrbiMed Private Investments II (QP), LP ("OPI II QP") and may be deemed to be indirectly beneficially owned by GP II, Advisors and Isaly. GP II is the sole general partner of OPI II QP. Advisors is the sole managing member of GP II. Isaly is the managing member of, and owner of a controlling interest in, Advisors. By virtue of such relationships, GP II, Advisors and Isaly may be deemed to have voting and investment power over the securities held by OPI II QP noted above. Accordingly, GP II, Advisors and Isaly may be deemed to have beneficial ownership of such securities.

4. This report on Form 3 is jointly filed by GP II, Advisors and Isaly. GP II, Advisors and Isaly have designated a representative, currently Carl L. Gordon, a member of Advisors, to serve on the Issuer's board of directors. Each of GP II, Advisors, Isaly and Carl L. Gordon disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its or his pecuniary interest therein, if any. This report shall not be deemed an admission that any of them is the beneficial owner of such securities for purposes of Section 16 of the Exchange Act or for any other purpose.

5. The Series A Convertible Preferred Stock, Series B Convertible Preferred Stock, Series C Convertible Preferred Stock, Series D Convertible Preferred Stock, Series E Convertible Preferred Stock and Series F Convertible Preferred Stock are each convertible into Common Stock at any time at the holder's election and will automatically convert upon the consummation of a qualified public offering of the registrant.

6. Each share of preferred stock is convertible into Common Stock at a ratio of 1 share of Common Stock per share of preferred stock, except for the Series E Convertible Preferred Stock, which may convert into Common Stock at a ratio greater than 1-to-1, based on a formula driven by the date on which the issuer completes an IPO and the price of such offering, as described at page 37 of the issuer's Amended Form S-1 registration statement filed September 6, 2013, File No. 333-190417. The conversion ratio used for the Series E Convertible Preferred Stock is based on an assumed closing date of September 24, 2013. There is no expiration date and no additional consideration is required upon conversion for any series of convertible preferred stock.

7. The warrants to purchase Common Stock are exercisable at any time at the holder's election and have an expiration date on June 10, 2020.

8. The warrants to purchase Common Stock are exercisable at any time at the holder's election and have an expiration date on July 9, 2020.

Remarks:

	/s/ Samuel D. Isaly	09/18/2013
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 5 (b)(v).
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