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* THIRD ROCK VENTURES LP (Last) (First) (Middle) 29 NEWBURY STREET (Street) BOSTON MA 02116 (City) (State) (Zip)	2. Date of Event Requiring Statement (Month/Day/Year) 06/18/2013	3. Issuer Name and Ticker or Trading Symbol bluebird bio, Inc. [ BLUE ]
		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)

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)
	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Series B Convertible Preferred Stock	(1)	(1)	Common Stock	3,394,194	(1)	D(2)
Series C Convertible Preferred Stock	(3)	(3)	Common Stock	758,121	(3)	D(2)
Series D Convertible Preferred Stock	(4)	(4)	Common Stock	581,933	(4)	D(2)

1. Name and Address of Reporting Person* THIRD ROCK VENTURES LP (Last) (First) (Middle) 29 NEWBURY STREET (Street) BOSTON MA 02116 (City) (State) (Zip)

1. Name and Address of Reporting Person* Third Rock Ventures GP, L.P. (Last) (First) (Middle) 29 NEWBURY STREET (Street) BOSTON MA 02116 (City) (State) (Zip)

1. Name and Address of Reporting Person* TRV GP, LLC (Last) (First) (Middle) 29 NEWBURY STREET (Street) BOSTON MA 02116 (City) (State) (Zip)

Explanation of Responses:

1. The Series B Convertible Preferred Stock is convertible into Common Stock on an 18.967-for-one basis into the number of shares of Common Stock as shown in column 3 at any time at the holder's election, and automatically upon the closing of the Issuer's initial public offering, and has no expiration date.

2. The general partner of Third Rock Ventures, L.P. is Third Rock Ventures GP, LP ("TRV GP"). The general partner of TRV GP is Third Rock Ventures GP, LLC ("TRV GP, LLC"). TRV GP and TRV GP, LLC disclaim beneficial ownership of such shares, except to the extent of any pecuniary interest therein.

3. The Series C Convertible Preferred Stock is convertible into Common Stock on an 18.967-for-one basis into the number of shares of Common Stock as shown in column 3 at any time at the holder's election, and automatically upon the closing of the Issuer's initial public offering, and has no expiration date.

4. The Series D Convertible Preferred Stock is convertible into Common Stock on an 18.967-for-one basis into the number of shares of Common Stock as shown in column 3 at any time at the holder's election, and automatically upon the closing of the Issuer's initial public offering, and has no expiration date.

Remarks:

	By: Third Rock Ventures GP, L.P., its general partner By: TRV GP, LLC, its general partner By: /s/ Kevin Gillis Kevin Gillis, Manager	06/18/2013
	By: TRV GP, LLC, its general partner By: /s/ Kevin Gillis, Manager	06/18/2013
	By: /s/ Kevin Gillis, Manager	06/18/2013
	** 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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