Offer letter, dated June 19, 2020, by and between the registrant and Matthew Norkunas, as amended July 6, 2020

Start Date: Your employment will begin on July 22, 2020 (the “Start Date”).

Salary: Generation Bio will pay you an annual Base Salary of $400,000, payable in accordance with Generation Bio’s standard payroll schedule and subject to applicable deductions and withholdings. This salary will be subject to periodic review and adjustments at Generation Bio’s discretion. Because this is an exempt position, you will not be eligible for any overtime pay.

Bonus: During the term of your employment with Generation Bio, you will be eligible for an annual incentive bonus (“Bonus”) for each fiscal year of your employment with Generation Bio. The amount, terms and conditions of such bonus are to be determined at the sole discretion of the Board of Directors of Generation Bio (the “Board”), and such terms may be changed and conditions may be changed at any time with or without notice to you. Your target annual Bonus shall be up to 40% of your annual salary. Your actual Bonus percentage is discretionary and will be subject to Generation Bio’s assessment of your performance as well as the performance of Generation Bio during the applicable

Incentive Equity Grant: You will be eligible to participate in the Company’s stock incentive program. Subject to the approval of the Board, you will be granted options to purchase 330,000 shares of Generation Bio’s common stock (the “Option Grant”). The options subject to the Option Grant (“Options”) will vest as to 25% of the underlying shares on the first anniversary of the Start Date and will vest as to the balance in equal quarterly installments of 6.25% thereafter until the fourth anniversary of the Start Date. The Option Grant will be subject to the terms and conditions of a written stock option agreement which you will be required to sign, and/or the Company’s written stock plan (the “Grant Documents”). The Options shall have an exercise price per share equal to the fair market value of the Company’s common stock at the time of grant, as determined by the Board.

Benefits: You may participate in the benefit programs offered by the Company to its employees, provided that you are eligible under and subject to all provisions of the plan documents that govern those programs. Benefits are subject to change at any time in the Company’s sole discretion. You will also be entitled to paid vacation and sick leave each year in accordance with the terms and conditions set forth in the Company’s policies. You will also be entitled to receive reimbursement for all reasonable business expenses incurred by you in performing your services to the Company in keeping with Company policies. In addition, you may be eligible for other benefits offered by the Company as set forth in the Employee Guide, which can be accessed on the Company’s intranet portal.

Accrued Obligations: If your employment terminates for any reason, including for Cause or as a result of Involuntary Termination, the Company will pay you the Accrued Obligations earned through your last day of employment (the “Separation Date”) on or before the time required by law or applicable policy, except to the extent any such payments would accelerate compensation in a manner inconsistent with compliance with Section 409A of the Internal Revenue Code of 1986, as amended (the “Code”).

Severance Benefits: If you are subject to an Involuntary Termination, in addition to the Accrued Obligations, you will be eligible to receive severance benefits as described under Sections 2(A) or (B) below, as applicable (“Severance Benefits”), provided you have: (i) returned all Company property in your possession on or prior to the Separation Date, (ii) resigned as a member of the Board of Directors of the Company (the “Board”) or of any subsidiary of the Company, to the extent you are then a Director of the Company or of any such subsidiary, and (iii) entered into a separation agreement that has become enforceable and irrevocable and includes a general release of all claims you may have against the Company or persons affiliated with the Company (the “Separation Agreement”). Notwithstanding the foregoing, except as set forth in Section 8(B)(iv), no term of this Agreement or the Separation Agreement shall impact or affect in any way your rights with respect to, and the Separation Agreement shall not include a waiver or release of any claims related to: (x) your status as a stockholder or equity holder of the Company or any rights you have under the terms of any equity award agreement between you and the Company, including any claims with respect to any options or other equity awards owned or held by you at the time your employment is terminated, or (y) any rights to indemnification from the Company, pursuant to any applicable governing documents of the Company or any applicable written agreement between you and the Company, rights under ERISA or rights which, as a matter of law, cannot be waived. The Separation Agreement, in a form to be provided to you by the Company, must be executed and become enforceable and irrevocable within 52 days following the Separation Date, or such shorter period of time prescribed by the Company (the date by which the Separation Agreement must become enforceable and irrevocable, the “Prescribed Deadline”). If the Separation Agreement is not executed or is executed but has not become enforceable and irrevocable by the Prescribed Deadline, you shall be entitled to the Accrued Obligations only and not to any Severance Benefits. Any Severance Benefits shall be paid, or begin to be paid, in the first regular payroll beginning after the Separation Agreement becomes enforceable and irrevocable, provided that if the foregoing 52-day period begins in the year in which your Separation Date occurs and ends in the following year, the Company will not make any payments before the first payroll date falling after the later of (A) January 1 of the year following the year in which your Separation Date occurs and (B) the date on which Separation Agreement becomes enforceable and irrevocable (the date the Severance Benefits are paid or commence pursuant to this sentence, the “Payment Date”). The first payroll shall include, however, all amounts that would otherwise have been paid to you between the Separation Date and your receipt of the first payment.

Involuntary Termination Prior to or More than 12 months Following a Change in Control In the event of your Involuntary Termination prior to or more than 12 months following a Change in Control, you are eligible to receive the following Severance Benefits:

Salary. The Company shall continue to pay you your base salary as in effect on the Separation Date for a period of nine (9) months, payable in accordance with the Company’s standard payroll schedule;

Bonus Payment. The Company may pay you an amount determined by reference to your target annual incentive bonus for the year in which the Separation Date occurs, based on Company and individual performance for such year, as determined by the Board in its sole discretion, and pro-rated based on the number of days you were employed by the Company in such year, which payment (if any) will be paid to you in a single lump sum amount on the later of (1) the Payment Date or (2) the date on which bonuses are paid to other executives of the Company but no later than two and one-half (2.5) months following the end of the year in which the Separation Date occurs; and

Health Insurance. If you are participating in the Company’s group health plan immediately prior to the Separation Date and you are eligible for and timely elect COBRA health and dental continuation, then the Company will continue to pay the share of the premium the Company would have paid to provide health and dental coverage to you and your eligible dependents if you had remained employed by the Company for a period ending on the earlier of the date that is nine (9) months following the Separation Date or the date COBRA eligibility ends, as such premiums become due, provided that the Company’s payment for COBRA coverage shall only apply if and while permitted under applicable tax or other laws as nondiscriminatory.

Involuntary Termination On or Within 12 Months Following a Change in Control

Salary. The Company shall pay you a lump sum amount on the Payment Date equal to twelve (12) months of your base salary as in effect on the Separation Date;

Bonus Payment. The Company shall pay you a lump sum amount on the Payment Date equal to your target annual incentive bonus for the year in which your Separation Date occurs;

Health Insurance. If you are participating in the Company’s group health plan immediately prior to the Separation Date and you are eligible for and timely elect COBRA health and dental continuation, then the Company will continue to pay the share of the premium the Company would have paid to provide health and dental coverage to you and your eligible dependents if you had remained employed by the Company for a period ending on the earlier of the date that is twelve (12) months following the Separation Date or the date COBRA eligibility ends, as such premiums become due, provided that the Company’s payment for COBRA coverage shall only apply if and while permitted under applicable tax or other laws as nondiscriminatory; and iv. Equity. One hundred percent (100%) of the unvested portion of any then-outstanding Equity Grant shall vest and become fully exercisable or non-forfeitable as of the Separation Date, provided however, that:

no such acceleration shall be effective until the Separation Agreement has become enforceable and irrevocable; and

if the Separation Agreement does not become enforceable and irrevocable, the portions of the Equity Grants that would have vested as a result of this provision shall not vest and shall be treated in accordance with the terms and conditions of the applicable award agreement.

Representation Regarding Other Obligations. Your employment is contingent upon your signing the Company’s Invention, Non-Disclosure, Non-Competition and Non-Solicitation Agreement (the “Non-Disclosure Agreement”). Further, you hereby represent to the Company that you are not a party to any agreement of any type which may impact or limit your ability to become employed by or perform your job at the Company or which is in any way inconsistent with the terms of this offer letter. You represent and agree that you will not disclose to the Company, use, or induce the Company to use any confidential or proprietary information or material belonging to any current or previous employer or others. Further, you hereby represent that (i) your employment with the Company and this offer letter does not and will not violate or conflict with any obligations you may have to or any agreements you may have with any former employer and (ii) you have provided the Company with all written agreements that describe any continuing postemployment obligations to any former employer.

Taxes

Withholding.

Payments Subject to Section 409A.

It is intended that each installment of the severance payments provided under this Agreement shall be treated as a separate “payment” for purposes of Section 409A of the Code (“Section 409A”). Neither the Company nor you shall have the right to accelerate or defer the delivery of any such payments except to the extent specifically permitted or required by Section 409A.

If, as of the date of your Separation from the Company, you are not a “specified employee” (within the meaning of Section 409A), then each installment of the severance payments shall be made on the dates and terms set forth in this Agreement.

If, as of the date of your Separation from the Company, you are a “specified employee” (within the meaning of Section 409A), then:

Each installment of the severance payments due under this Agreement that, in accordance with the dates and terms set forth herein, will in all circumstances, regardless of when your Separation occurs, be paid within the short-term deferral period (as defined under Section 409A) shall be treated as a short-term deferral within the meaning of Treasury Regulation Section 1.409A-1(b)(4) to the maximum extent permissible under Section 409A and shall be paid on the dates and terms set forth this Agreement; and

Each installment of the severance payments due under this Agreement that is not described in this Section 3(B)(iii)(1) and that would, absent this subsection, be paid within the six-month period following your Separation from the Company shall not be paid until the date that is six months and one day after such Separation (or, if earlier, as soon as practicable following your death), with any such installments that are required to be delayed being accumulated during the six-month period and paid in a lump sum on the date that is six months and one day following your Separation and any subsequent installments, if any, being paid in accordance with the dates and terms set forth herein; provided, however, that the preceding provisions of this sentence shall not apply to any installment of payments if and to the maximum extent that that such installment is deemed to be paid under a separation pay plan that does not provide for a deferral of compensation by reason of the application of Treasury Regulation 1.409A-1(b)(9)(iii) (relating to separation pay upon an involuntary separation from service). Any installments that qualify for the exception under Treasury Regulation Section 1.409A-1(b)(9)(iii) must be paid no later than the last day of your second taxable year following the taxable year in which the Separation occurs.

Interpretation, Amendment and Enforcement. This offer letter, along with the Non-Disclosure Agreement and the Grant Documents, constitute the complete agreement between you and the Company, contain all the terms of your employment, and supersede any prior agreements, representations or understandings (whether written, oral or implied) between you and the Company. The terms of this offer letter and the resolution of any disputes as to the meaning, effect, performance or validity of this offer letter or arising out of, related to, or in any way connected with, this offer letter, your employment with the Company or any other relationship between you and the Company (the “Disputes”) will be governed by Massachusetts law, excluding laws relating to conflicts or choice of law. You and the Company submit to the exclusive personal jurisdiction of the federal and state courts located in the Commonwealth of Massachusetts in connection with any Dispute or any claim related to any Dispute.

Other Terms. This offer letter shall not be construed as an agreement, either express or implied, to employ you for any stated term, and shall in no way alter the Company’s policy of employment at-will, which means that you have the right to terminate your employment relationship with the Company at any time for any reason and the Company has the right to terminate its employment relationship with you at any time for any reason, with or without cause or notice. Similarly, nothing in this letter shall be construed as an agreement, either express or implied, to pay you any compensation or grant you any benefit beyond the end of your employment with the Company, except as may be required by, and subject to the conditions set forth in, Section 8. You have the right to consult with counsel before signing this offer letter.

Definitions: The following terms have the meaning set forth below wherever they are used in this letter:

General Provisions

Signature

Name: Matthew Norkunas

Dated: July 6, 2020