Pricing Information Provided Orally by Underwriters Public offering price: $76.00 per share Number of Underwritten Shares: 3,289,473 Number of Option Shares: 493,420 Exhibit A FORM OF LOCK-UP AGREEMENT , 2016 Xxxxxxx, Sachs & Co. Xxxxxxx Lynch, Pierce, Xxxxxx & Xxxxx Incorporated Xxxxx and Company, LLC as Representatives of the several Underwriters listed in Schedule 1 to the Underwriting Agreement referred to below c/o Goldman, Xxxxx & Co. 000 Xxxx Xxxxxx Xxx Xxxx, Xxx Xxxx, 00000 c/o Merrill Lynch, Pierce, Xxxxxx & Xxxxx Incorporated Xxx Xxxxxx Xxxx Xxx Xxxx, Xxx Xxxx, 00000 c/o Cowen and Company, LLC 000 Xxxxxxxxx Xxxxxx Xxx Xxxx, Xxx Xxxx 00000 Re: bluebird bio, Inc. — Public Offering Ladies and Gentlemen: The undersigned understands that you, as Representatives of the several Underwriters, propose to enter into an Underwriting Agreement (the “Underwriting Agreement”) with bluebird bio, Inc., a Delaware corporation (the “Company”), providing for the public offering (the “Public Offering”) by the several Underwriters named in Schedule 1 to the Underwriting Agreement (the “Underwriters”), of common stock, par value $0.01 per share, of the Company (the “Common Stock”). Capitalized terms used herein and not otherwise defined shall have the meanings set forth in the Underwriting Agreement. In consideration of the Underwriters’ agreement to purchase and make the Public Offering of the Common Stock, and for other good and valuable consideration receipt of which is hereby acknowledged, the undersigned hereby agrees that, without the prior written consent of Xxxxxxx, Sachs & Co., Xxxxxxx Lynch, Pierce, Xxxxxx & Xxxxx Incorporated and Xxxxx and Company, LLC on behalf of the Underwriters, the undersigned will not, during the period ending 45 days after the date of the final prospectus supplement relating to the Public Offering (the “Prospectus”), (1) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock (including without limitation, Common Stock or such other securities which may be deemed to be beneficially owned by the undersigned in accordance with the rules and regulations of the Securities and Exchange Commission and securities which may be issued upon exercise of a stock option or warrant), or publicly disclose the intention to make any offer, sale, pledge or disposition, (2) enter into any swap or other agreement that transfers, in whole or in part, any of the economic consequences of ownership of the Common Stock or such other securities, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise or (3) make any demand for or exercise any right with respect to the registration of any shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock, in each case other than (A) transfers of shares of Common Stock or other securities of the Company as a bona fide gift or gifts, (B) transfers of shares of Common Stock or other securities of the Company to a trust or limited family partnership for the direct or indirect benefit of the undersigned or the immediate family of the undersigned, (C) transfers of shares of Common Stock or other securities of the Company by will, other testamentary document or intestate succession to the legal representative, heir, beneficiary or a member of the immediate family of the undersigned in a transaction not involving a disposition for value, (D) transactions relating to shares of Common Stock acquired in open market transactions after the completion of the Public Offering, provided that no filing under Section 16(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) shall be required or shall be made during the 45 day period after the date of the Prospectus in connection with subsequent sales of Common Stock acquired in such open market transactions, (E) transfers of Common Stock pursuant to a trading plan established pursuant to Rule 10b5-1 under the Exchange Act prior to the date hereof, which trading plan shall not be amended during the 45 days after the date of the prospectus but may be terminated during the 45 days after the date of the prospectus and (F) the establishment of a trading plan pursuant to Rule 10b5-1 under the Exchange Act providing for the transfer of shares of Common Stock, to the extent that such plan does not provide for the transfer of shares of Common Stock during the 45 days after the date of the Prospectus; provided that in the case of any transfer or distribution pursuant to clause (A), (B) or (C), each transferee, donee or distributee shall execute and deliver to the Representatives a lock-up letter in the form of this paragraph; and provided, further, that in the case of any transfer or distribution pursuant to clause (A), (B) or (C), no filing by any party (donor, donee, transferor or transferee) under the Exchange Act or other public announcement shall be required or shall be made voluntarily in connection with such transfer or distribution (other than a filing on a Form 5 made after the expiration of the 45 day period referred to above). [Notwithstanding the foregoing, the undersigned may transfer or sell shares of Common Stock for the sole purpose of satisfying any tax withholding obligations incurred upon the vesting of any restricted stock units that vest during the 45-day period referred to above, through the surrender to the Company of shares of Common Stock or through sales of shares of Common Stock in the market, provided that any shares of Common Stock received by the undersigned upon such vesting shall be subject to the restrictions provided for in this Letter Agreement; provided, that any related filing under Section 16(a) of the Exchange Act must note that such transfer or sale was made solely to satisfy tax withholding obligations.] In furtherance of the foregoing, the Company, and any duly appointed transfer agent for the registration or transfer of the securities described herein, are hereby authorized to decline to make any transfer of securities if such transfer would constitute a violation or breach of this Letter Agreement. The undersigned hereby represents and warrants that the undersigned has full power and authority to enter into this Letter Agreement. All authority herein conferred or agreed to be conferred and any obligations of the undersigned shall be binding upon the successors, assigns, heirs or personal representatives of the undersigned. The undersigned understands that, if the Underwriting Agreement does not become effective, or if the Underwriting Agreement (other than the provisions thereof which survive termination) shall terminate or be terminated prior to payment for and delivery of the Common Stock to be sold thereunder, the undersigned shall be released from, all obligations under this Letter Agreement. The undersigned understands that the Underwriters are entering into the Underwriting Agreement and proceeding with the Public Offering in reliance upon this Letter Agreement. This Letter Agreement and any claim, controversy or dispute arising under or related to this Letter Agreement shall be governed by and construed in accordance with the laws of the State of New York, without regard to the conflict of laws principles thereof. Very truly yours, [NAME OF STOCKHOLDER] By: Name:
