GBIO Generation Bio Company

Filed by the

Filed by a Party other than the

Registrant  ☒

​

Registrant  ☐

​

☐

Preliminary Proxy Statement

☐

Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐

Definitive Proxy Statement

☒

Definitive Additional Materials

☐

Soliciting Material under §240.14a-12

☒

No fee required.

☐

Fee computed on table below per Exchange Act Rules 14a-6(i)(1) and 0-11.

(1)

Title of each class of securities to which transaction applies:

​

(2)

Aggregate number of securities to which transaction applies:

​

(3)

Per unit price or other underlying value of transaction computed pursuant to Exchange Act Rule 0-11 (set forth the amount on which the filing fee is calculated and state how it was determined):

​

(4)

Proposed maximum aggregate value of transaction:

​

(5)

Total fee paid:

​

☐

Fee paid previously with preliminary materials.

☐

Check box if any part of the fee is offset as provided by Exchange Act Rule 0-11(a)(2) and identify the filing for which the offsetting fee was paid previously. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

(1)

Amount Previously Paid:

​

(2)

Form, Schedule or Registration Statement No.:

​

(3)

Filing Party:

​

(4)

Date Filed:

​

Your Vote Counts! GENERATION BIO CO. 2021 Annual Meeting Vote by June 8, 2021 11:59 PM ET GENERATION BIO CO. 301 BINNEY STREET CAMBRIDGE, MA 02142 D36858-P49278 You invested in GENERATION BIO CO. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy material for the stockholder meeting to be held on June 9, 2021. Get informed before you vote View the Notice and Proxy Statement and Annual Report online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 26, 2021. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. Smartphone users Point your camera here and vote without entering a control number Vote Virtually at the Meeting* June 9, 2021 8:00 AM, EDT Virtually at: www.virtualshareholdermeeting.com/GBIO2021 *Please check the meeting materials for any special requirements for meeting attendance. V1 For complete information and to vote, visit www.ProxyVote.com Control #

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Board Recommends Voting Items D36859-P49278 Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Sign up for E-delivery”. 1. Election of three Class I directors to our board of directors, each to serve until the 2024 Annual Meeting of Stockholders; Nominees: 01) Jeffrey Jonas, M.D. 02) Geoff McDonough, M.D. 03) Donald Nicholson, Ph.D. For 2. Ratification of the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2021. For NOTE: In their discretion, the proxyholders are authorized to vote upon such other business as may properly come before the meeting and any adjournment or postponement thereof.