THE INSTRUCTIONS ACCOMPANYING THIS LETTER OF TRANSMITTAL SHOULD BE READ CAREFULLY BEFORE THIS LETTER OF TRANSMITTAL IS COMPLETED. REQUESTS FOR ASSISTANCE OR ADDITIONAL COPIES OF THE OFFER TO PURCHASE AND THIS LETTER OF TRANSMITTAL MAY BE MADE TO OR OBTAINED FROM THE INFORMATION AGENT AT ITS ADDRESS OR TELEPHONE NUMBER SET FORTH BELOW.

If the certificate(s) representing Shares (as defined below) to be tendered have been mutilated, lost, stolen or destroyed, stockholders should contact 2seventy bio, Inc.’s transfer agent, Equiniti Trust Company, LLC, immediately by calling (800) 937-5449 or (718) 921-8124. This Letter of Transmittal and related documents cannot be processed until the procedures for replacing such certificate(s) have been followed. You may be required to post a bond to secure against the risk that the Share certificate(s) may be subsequently recirculated. See Instruction 9.

You must sign this Letter of Transmittal in the appropriate space provided below, with signature guarantee if required, and complete the enclosed IRS Form W-9 or provide the appropriate IRS Form W-8.

The Offer (as defined below) is being made to all holders of Shares. We are not aware of any jurisdiction in which the making of the Offer or the acceptance thereof would be prohibited by securities, “blue sky” or other valid laws of such jurisdiction. If we become aware of any U.S. state in which the making of the Offer or the acceptance of Shares pursuant thereto would not be in compliance with an administrative or judicial action taken pursuant to a U.S. state statute, we will make a good faith effort to comply with any such law. If, after such good faith effort, we cannot comply with any such law, the Offer will not be made to, (nor will tenders be accepted from or on behalf of), the holders of Shares in any U.S. or foreign jurisdiction in which the making of the Offer or acceptance thereof would not be in compliance with the laws of such jurisdiction. In any jurisdictions where the applicable laws require that the Offer be made by a licensed broker or dealer, the Offer will be deemed to be made on behalf of Purchaser by one or more registered brokers or dealers licensed under the laws of such jurisdiction to be designated by Purchaser.

This Letter of Transmittal is to be used by stockholders of 2seventy bio, Inc. (a) if certificates are to be forwarded herewith or (b) if Shares are held in book-entry form on the records of the Depositary.

Holders of outstanding Shares, whose certificates for such Shares are not immediately available or who cannot deliver such certificates and all other required documents to the Depositary at or prior to the Expiration Time (as defined below) or who cannot complete the procedure for book-entry transfer at or prior to the Expiration Time, may only tender their Shares according to the guaranteed delivery procedure set forth in Section 3 of the Offer to Purchase. See Instruction 2. Delivery of documents to The Depository Trust Company (the “Book-Entry Transfer Facility” or “DTC”) does not constitute delivery to the Depositary.

The undersigned hereby tenders to Daybreak Merger Sub Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Bristol-Myers Squibb Company, a Delaware corporation (“Parent”), the above-described shares of common stock, par value $0.0001 per share (the “Shares”), of 2seventy bio, Inc., a Delaware corporation (“2seventy bio”), pursuant to Purchaser’s offer to acquire all of the outstanding Shares for $5.00 per Share, in cash, without interest, subject to any applicable withholding of taxes, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated April 14, 2025 (together with any amendments or supplements thereto, the “Offer to Purchase”), receipt of which is hereby acknowledged, and in this Letter of Transmittal (which, together with any amendments or supplements thereto, collectively constitute the “Offer”). The Offer expires one minute following 11:59 p.m., New York City Time, on May 12, 2025, unless extended or earlier terminated as permitted by the Merger Agreement (as defined below) (such time or such subsequent time to which the expiration of the Offer is extended in accordance with the Merger Agreement, the “Expiration Time”). To the extent permitted under the Merger Agreement, Purchaser reserves the right to assign to a controlled affiliate of Parent the right to purchase Shares tendered pursuant to the Offer (provided that such assignment shall not prevent, impede or delay the consummation of the Offer or the other transactions contemplated by the Merger Agreement or otherwise impede the rights of the stockholders of 2seventy bio under the Merger Agreement, or relieve Parent of its obligations under the Merger Agreement).

Upon the terms and subject to the conditions of the Offer (including, if the Offer is extended or amended, the terms and conditions of such extension or amendment), and effective upon acceptance for payment for the Shares validly tendered herewith in accordance with the terms of the Offer, the undersigned hereby sells, assigns and transfers to, or upon the order of, Purchaser all right, title and interest in and to all of the Shares that are being tendered hereby. In addition, the undersigned hereby irrevocably appoints Equiniti Trust Company, LLC as the depositary for the Offer (the “Depositary”) and the true and lawful agent and attorney-in-fact and proxy of the undersigned with respect to such Shares, with full power of substitution (such proxy and power of attorney being deemed to be an irrevocable power coupled with an interest in the Shares tendered by this Letter of Transmittal), to (a) deliver certificates for such Shares, or transfer ownership of such Shares on the account books maintained by the Book-Entry Transfer Facility, together, in any such case, with all accompanying evidences of transfer and authenticity, to or upon the order of Purchaser, (b) present such Shares for transfer on the books of 2seventy bio and (c) receive all benefits and otherwise exercise all rights of beneficial ownership of such Shares, all in accordance with the terms of the Offer.

The undersigned hereby irrevocably appoints each of the designees of Purchaser as the attorneys-in-fact and proxies of the undersigned, each with full power of substitution, to exercise all voting and other rights of the undersigned in such manner as each such attorney-in-fact and proxy or his or her substitute shall in his or her sole discretion deem proper, with respect to all of the Shares validly tendered hereby which have been accepted for payment by Purchaser prior to the time of any vote or other action, at any meeting of stockholders of 2seventy bio (whether annual or special and whether or not an adjourned meeting), by written consent or otherwise. This proxy and power of attorney is irrevocable and is granted in consideration of, and is effective upon, the acceptance for payment of such Shares by Purchaser in accordance with the terms of the Offer. Such acceptance for payment shall revoke any other proxies, powers of attorney, or written consent granted by the undersigned at any time with respect to such Shares, and no subsequent proxies or powers of attorney will be given, or written consents will be executed by the undersigned (and if given or executed, will not be deemed to be effective). Purchaser reserves the right to require that, in order for Shares to be deemed validly tendered, immediately upon Purchaser’s acceptance for payment of such Shares, Purchaser or its designees must be able to exercise full voting, consent and other rights with respect to such Shares, including voting at any meeting of 2seventy bio’s stockholders.

The undersigned hereby represents and warrants that the undersigned has full power and authority to tender, sell, assign and transfer the Shares tendered herein and that when the same are accepted for payment by Purchaser, Purchaser will acquire good, marketable and unencumbered title thereto, free and clear of all liens, restrictions, charges and encumbrances and that the same will not be subject to any adverse claims. The undersigned hereby represents and warrants that the undersigned is the registered owner of the Shares, or the Share certificate(s) have been endorsed to the undersigned in blank, or the undersigned is a participant in the

Book-Entry Transfer Facility whose name appears on a security position listing as the owner of the Shares. The undersigned will, upon request, execute and deliver any additional documents deemed by the Depositary or Purchaser to be reasonably necessary to complete the sale, assignment and transfer of the Shares tendered hereby.

All authority herein conferred or agreed to be conferred shall not be affected by, and shall survive, the death or incapacity of the undersigned, and any obligation of the undersigned hereunder shall be binding upon the heirs, executors, administrators, personal representatives, trustees in bankruptcy, successors and assigns of the undersigned. Except as stated in the Offer, this tender is irrevocable.

The undersigned hereby acknowledges that delivery of any Share certificate(s) shall be effected, and risk of loss and title to such Share certificate(s) shall pass, only upon the proper delivery of such Share certificate(s) or transfer of the uncertificated Shares represented by book entry to the Depositary.

The undersigned understands that tenders of Shares pursuant to any one of the procedures described in Section 3 of the Offer to Purchase and in the Instructions hereto will constitute an agreement between the undersigned and Purchaser upon the terms and subject to the conditions of the Offer. Without limiting the foregoing, if the price to be paid in the Offer is amended in accordance with the terms of the Agreement and Plan of Merger, dated as of March 10, 2025 (as it may be amended or supplemented from time to time, the “Merger Agreement”), among Parent, Purchaser and 2seventy bio pursuant to which the Offer is being made, the price to be paid to the undersigned will be the amended price notwithstanding the fact that a different price is stated in this Letter of Transmittal.

Unless otherwise indicated under “Special Payment Instructions,” please issue the check for the purchase price of any Shares purchased, and, if appropriate, return any certificates evidencing Shares not tendered or not accepted for payment in the name(s) of the undersigned. Similarly, unless otherwise indicated under “Special Delivery Instructions,” please mail the check for the purchase price of any Shares purchased and any certificates evidencing Shares not tendered or not accepted for payment (and accompanying documents, as appropriate) to the undersigned at the address shown below the undersigned’s signature(s). In the event that both “Special Payment Instructions” and “Special Delivery Instructions” are completed, please issue the check for the purchase price of any Shares purchased and return any Shares not tendered or not accepted for payment in the name(s) of, and mail said check and any certificates to, the person(s) so indicated. The undersigned recognizes that Purchaser has no obligation, pursuant to the “Special Payment Instructions,” to transfer any Shares from the name of the registered holder(s) thereof if Purchaser does not accept for payment any of the Shares so tendered.

To be completed ONLY if the check for the purchase price of Shares purchased (less the amount of any federal income and backup withholding tax required to be withheld) is to be issued in the name of someone other than the undersigned.

To be completed ONLY if the check for the purchase price of Shares purchased (less the amount of any federal income and backup withholding tax required to be withheld) is to be mailed to someone other than the undersigned or to the undersigned at an address other than that shown below the undersigned’s signature(s).

1. Guarantee of Signatures. Except as otherwise provided below, all signatures on this Letter of Transmittal must be guaranteed by a financial institution (including most banks, savings and loan associations and brokerage houses) that is a member of a recognized Medallion Program approved by The Securities Transfer Association, Inc., including the Securities Transfer Agents Medallion Program (STAMP), the Stock Exchange Medallion Program (SEMP) and the New York Stock Exchange, Inc. Medallion Signature Program (MSP) or any other “eligible guarantor institution” (as such term is defined in Rule 17Ad-15 under the Securities Exchange Act of 1934, as amended) (each, an “Eligible Institution”). Signatures on this Letter of Transmittal need not be guaranteed (a) if this Letter of Transmittal is signed by the registered holder(s) of the Shares (which term, for purposes of this document, shall include any participant in the Book-Entry Transfer Facility whose name appears on a security position listing as the owner of Shares, trustees, executors, administrators, guardians, attorney-in-fact, officers of a corporation or other persons acting in a fiduciary or representative capacity, see Instruction 5), tendered herewith and such holder(s) have not completed the box entitled “Special Payment Instructions” on this Letter of Transmittal or (b) if such Shares are tendered for the account of an Eligible Institution. See Instruction 5.

2. Delivery of Letter of Transmittal and Shares. This Letter of Transmittal is to be used either if certificates are to be forwarded herewith or Shares are held in book-entry form on the records of the Depositary. Certificates for all physically delivered Shares, as well as a properly completed and duly executed Letter of Transmittal, together with any required signature guarantees and any other documents required by this Letter of Transmittal, must be received by the Depositary at one of its addresses set forth on the front page of this Letter of Transmittal by the Expiration Time.

Stockholders whose certificates for Shares are not immediately available or stockholders who cannot deliver their certificates and all other required documents to the Depositary by the Expiration Time or who cannot comply with the procedures for book-entry transfer by the Expiration Time may tender their Shares by completing a Notice of Guaranteed Delivery pursuant to the guaranteed delivery procedure set forth in Section 3 of the Offer to Purchase. Under the guaranteed delivery procedure:

The method of delivery of Shares, this Letter of Transmittal and all other required documents is at the election and sole risk of the tendering stockholder. Shares will be deemed delivered only when actually received by the Depositary. If certificates for Shares are sent by mail, we recommend registered mail with return receipt requested, properly insured, in time to be received on or prior to the Expiration Time. In all cases, sufficient time should be allowed to ensure timely delivery.

No alternative, conditional or contingent tenders will be accepted and no fractional shares will be purchased. By executing this Letter of Transmittal (or a manually signed facsimile thereof), the tendering stockholder waives any right to receive any notice of the acceptance for payment of the Shares.

3. Inadequate Space. If the space provided herein is inadequate, the certificate numbers and/or the number of Shares should be listed on a separate signed schedule attached hereto.

4. Partial Tenders. If fewer than all of the Shares represented by any certificate delivered to the Depositary are to be tendered, fill in the number of Shares which are to be tendered in the box entitled “Number of Shares Tendered.” In such case, a new certificate for the remainder of the Shares represented by the old certificate will be issued and sent to the person(s) signing this Letter of Transmittal, unless otherwise provided in the appropriate box on this Letter of Transmittal, as promptly as practicable following the expiration or

termination of the Offer. All Shares represented by certificates delivered to the Depositary will be deemed to have been tendered unless otherwise indicated. In the case of Shares tendered by book-entry transfer at DTC (or Shares held in a direct registration account maintained by 2seventy bio’s transfer agent), any tendered but unpurchased Shares (including as a result of any necessary proration) will be credited to the appropriate account maintained by the tendering stockholder at DTC (or by 2seventy bio’s transfer agent). In each case, Shares will be returned or credited without expense to the stockholder.

5. Signatures on Letter of Transmittal; Stock Powers and Endorsements. If this Letter of Transmittal is signed by the registered holder(s) of the Shares tendered hereby, the signature(s) must correspond with the name(s) as written on the face of the certificates without alteration or any change whatsoever.

If any of the Shares tendered hereby are held of record by two or more persons, all such persons must sign this Letter of Transmittal.

If any of the Shares tendered hereby are registered in different names on different certificates, it will be necessary to complete, sign and submit as many separate Letters of Transmittal as there are different registrations of certificates.

If this Letter of Transmittal is signed by the registered holder(s) of the Shares tendered hereby, no endorsements of certificates or separate stock powers are required unless payment of the purchase price is to be made, or Shares not tendered or not accepted for payment are to be returned, in the name of any person other than the registered holder(s). Signatures on any such certificates or stock powers must be guaranteed by an Eligible Institution.

If this Letter of Transmittal is signed by a person other than the registered holder(s) of the Shares tendered hereby, certificates must be endorsed or accompanied by appropriate stock powers, in either case, signed exactly as the name(s) of the registered holder(s) appear(s) on the certificates for such Shares. Signature(s) on any such certificates or stock powers must be guaranteed by an Eligible Institution.

If this Letter of Transmittal or any certificate or stock power is signed by a trustee, executor, administrator, guardian, attorney-in-fact, officer of a corporation or other person acting in a fiduciary or representative capacity, such person should so indicate when signing, and proper evidence satisfactory to Purchaser of the authority of such person so to act must be submitted, or in lieu of evidence, a Guarantee of Signature (see Instruction 1).

6. Stock Transfer Taxes. Purchaser will pay any stock transfer taxes incurred by it with respect to the sale and transfer of any Shares to Purchaser pursuant to the Offer; provided that the payment of any transfer, documentary, sales, use, stamp, registration, value added and other taxes and fees (including any penalties and interest) incurred by a holder of Shares in connection with the Merger or the other Transactions, and the filing of any related tax returns, shall be the sole responsibility of such holder.

7. Special Payment and Delivery Instructions. If the check for the purchase price of any Shares purchased is to be issued in the name of a person other than the person(s) signing this Letter of Transmittal or if the check is to be mailed to someone other than the person(s) signing this Letter of Transmittal or to the person(s) signing this Letter of Transmittal at an address other than that shown above, the appropriate boxes on this Letter of Transmittal should be completed.

8. Backup Withholding. Under the U.S. federal income tax laws, unless certain certification requirements are met, the Depositary generally will be required to withhold at the applicable backup withholding rate (currently 24%) from any payments made to a stockholder pursuant to the Offer. In order to avoid such backup withholding, each tendering stockholder, and, if applicable, each other payee, in each case that is a U.S. person (as defined in the instructions to IRS Form W-9) must provide the Depositary with such stockholder’s or payee’s correct taxpayer identification number and certify that such stockholder or payee is not subject to such backup withholding by completing the IRS Form W-9 enclosed herein. In general, if a stockholder or payee is an individual, the taxpayer identification number is the social security number of such individual. If the stockholder or payee does not provide the Depositary with its correct taxpayer identification number, the stockholder or payee may be subject to a penalty imposed by the Internal Revenue Service. Generally, foreign stockholders are not subject to these backup withholding and reporting requirements. In order to satisfy the Depositary that a stockholder who is not a U.S. person (as defined in the instructions to IRS Form W-9) is exempt, such stockholder or payee must submit to the Depositary a properly completed IRS Form W-8, signed under penalties of perjury, attesting to that stockholder’s foreign status. Such IRS Form W-8 can be obtained from the Depositary

or the Internal Revenue Service (www.irs.gov/formspubs/index.html). The instructions to the enclosed IRS Form W-9 contain further information concerning backup withholding and instructions for completing the IRS Form W-9 (including how to obtain a taxpayer identification number if you do not have one and how to complete the IRS Form W-9 if Shares are held in more than one name).

Failure to provide an IRS Form W-9 or the appropriate IRS Form W-8 will not, by itself, cause Shares to be deemed invalidly tendered, but may require the Depositary to withhold from any payments made pursuant to the Offer.

Backup withholding is not an additional U.S. federal income tax. Rather, the U.S. federal income tax liability of a person subject to backup withholding will be reduced by the amount of tax withheld. If withholding results in an overpayment of taxes, a refund may be obtained provided that the required information is timely furnished to the Internal Revenue Service. Failure to complete and provide an IRS Form W-9 or the appropriate IRS Form W-8 may result in U.S. federal backup withholding on any payments made to you pursuant to the Offer.

9. Mutilated, Lost, Stolen or Destroyed Certificates. If any certificate(s) representing Shares to be tendered have been mutilated, lost, stolen or destroyed, stockholders should contact 2seventy bio’s transfer agent, Equiniti Trust Company, LLC, immediately by calling (800) 937-5449 or (718) 921-8124. With respect to Shares represented by certificates, the stockholder will then be instructed as to the steps that must be taken in order to replace the certificate(s). This Letter of Transmittal and related documents cannot be processed until the procedures for replacing lost, mutilated, destroyed or stolen certificate(s) have been followed.

10. Requests for Assistance or Additional Copies. Requests for assistance or additional copies of the Offer to Purchase and this Letter of Transmittal may be obtained from the Information Agent at its address or telephone numbers set forth below.

11. Waiver of Conditions. Purchaser reserves the right to waive any of the specified conditions of the Offer in the case of any Shares tendered, subject in certain cases to the prior written consent of 2seventy bio, or make any other changes in the terms and conditions of the Offer not inconsistent with the terms of the Merger Agreement.

12. Irregularities. All questions as to Offer Price, the form of documents and the validity, eligibility (including time of receipt) and acceptance for payment of any tender of Shares will be determined by Purchaser in its sole discretion, which determination shall be final and binding on you. Purchaser reserves the absolute right to reject any or all tenders of Shares it determines not to be in proper form or the acceptance of which or payments for which may, in the opinion of Purchaser, be unlawful. Purchaser also reserves the absolute right to waive any defect or irregularity in the tender of any Shares by any particular stockholder, whether or not similar defects or irregularities are waived in the case of other stockholders. No tender of shares will be deemed to have been validly made until all defects and irregularities have been cured or waived to the satisfaction of Purchaser. None of 2seventy bio, Purchaser, Parent, the Depositary, the Information Agent (as defined in the Offer to Purchase) or any other person will be under any duty to give notification of any defects of irregularities in tenders or incur any liability or failure to give any such notifications.

IMPORTANT: This Letter of Transmittal (or a manually signed facsimile thereof) together with any signature guarantees and any other required documents, must be received by the Depositary on or prior to the Expiration Time and either certificates for tendered Shares must be received by the Depositary or Shares must be delivered pursuant to the procedures for book-entry transfer, in each case on or prior to the Expiration Time, or the tendering stockholder must comply with the procedures for guaranteed delivery.