GEOFF MCDONOUGH - 15 Oct 2025 Form 4 Insider Report for Generation Bio Co. (GBIO)

Role: PRESIDENT AND CEO, Director
Signature: /s/ Shawna-Gay White, Attorney-in-Fact
Issuer symbol: GBIO
Transactions as of: 15 Oct 2025
Net transactions value: -$1,040
Form type: 4
Filing time: 17 Oct 2025, 16:05:12 UTC
Previous filing: 17 Jul 2025
Next filing: 21 Nov 2025
SEC: View on sec.gov

Reporting Owners (1)

Name	Relationship	Address	Signature	Signature date	CIK
MCDONOUGH GEOFF	PRESIDENT AND CEO, Director	C/O GENERATION BIO CO., 301 BINNEY ST, CAMBRIDGE	/s/ Shawna-Gay White, Attorney-in-Fact	17 Oct 2025	0001732701

Transactions Table

Type	Sym	Class	Transaction	Value $	Shares	Change %	* Price $	Shares After	Date	Ownership	Footnotes
transaction	GBIO	Common Stock	Options Exercise		+527	+0.39%		136,991	15 Oct 2025	Direct	F1, F2
transaction	GBIO	Common Stock	Tax liability	$1,040	-155	-0.11%	$6.71	136,836	15 Oct 2025	Direct	F1
holding	GBIO	Common Stock						22,646	15 Oct 2025	By 2018 Trust	F1, F3
holding	GBIO	Common Stock						27,500	15 Oct 2025	By 2020 Trust	F1, F4

Derivative Securities (e.g., puts, calls, warrants, options, convertible securities)

Type	Sym	Class	Transaction	Value $	Shares	Change %	* Price $	Shares After	Date	Underlying Class	Amount	Exercise Price	Ownership	Footnotes
transaction	GBIO	Restricted Stock Units	Options Exercise	$0	-527	-17%	$0.000000	2,636	15 Oct 2025	Common Stock	527		Direct	F1, F2, F5

* An asterisk sign (*) next to the price indicates that the price is likely invalid.

Explanation of Responses:

Id	Content
F1	Amounts reported in this Form 4 reflect the one-for-10 reverse stock split effected by the issuer on July 21, 2025.
F2	Each restricted stock unit represents the right to receive one share of the company's common stock.
F3	The securities are directly held by the McDonough Family 2018 Irrevocable Trust, or the 2018 Trust. The reporting person is the settlor of the 2018 Trust, Allison L. McDonough and David S. Grayzel are trustees for the 2018 Trust, and the 2018 Trust is for the benefit of Graeme and Owen McDonough. The reporting person may be deemed to beneficially own the shares of common stock owned by the 2018 Trust.
F4	The securities are directly held by the McDonough Family 2020 Irrevocable Trust, or the 2020 Trust. The reporting person is the settlor of the 2020 Trust, Allison L. McDonough and David S. Grayzel are trustees for the 2020 Trust, and the 2020 Trust is for the benefit of Allison, Graeme and Owen McDonough. The reporting person may be deemed to beneficially own the shares of common stock owned by the 2020 Trust.
F5	On January 20, 2023, the reporting person was granted 8,437 restricted stock units. The shares underlying the restricted stock units vest over four years, with 25% of the shares vested on January 15, 2024 and the remaining shares vesting in equal quarterly installments thereafter.