Generation Bio Reports Business Highlights and Second Quarter 2022 Financial Results Cash balance of $312.2M expected to fund operations into 2025 Cambridge, Mass.

Generation Bio Reports Business Highlights and

Second Quarter 2022 Financial Results

Cash balance of $312.2M expected to fund operations into 2025

Cambridge, Mass. - Aug. 4, 2022 - Generation Bio Co. (Nasdaq: GBIO), a biotechnology company innovating genetic medicines for people living with rare and prevalent diseases, reported business highlights and second quarter 2022 financial results.

"We are advancing our leading non-viral genetic medicine platform to unlock the potential of durable, redosable non-viral DNA therapeutics, and we have made significant progress in establishing the proprietary technologies necessary to realize our vision," said Geoff McDonough, M.D., president and chief executive officer of Generation Bio. "This quarter, process development and scaling of rapid enzymatic synthesis (RES) for production of our closed-ended DNA (ceDNA) construct has allowed us to pursue a more flexible and capital efficient manufacturing strategy. In the remainder of the year, we plan to continue to develop our unique cell-targeted lipid nanoparticle (ctLNP) system for systemic delivery to hepatocytes, as well as for other therapeutic areas such as the retina and vaccines."

Second Quarter 2022 Financial Results

About Generation Bio

Generation Bio is innovating genetic medicines to provide durable, redosable treatments for people living with rare and prevalent diseases. The company's non-viral genetic medicine platform incorporates a novel DNA construct called closed-ended DNA, or ceDNA; a unique cell-targeted lipid nanoparticle delivery system, or ctLNP; and a highly scalable capsid-free manufacturing process that uses proprietary cell-free rapid enzymatic synthesis, or RES, to produce ceDNA. The platform is designed to enable multi-year durability from a single dose, to deliver large genetic payloads, including multiple genes, to specific cell types, and to allow titration and redosing to adjust or extend expression levels in each patient. RES has the potential to expand Generation Bio's manufacturing scale to hundreds of millions of doses to support its mission to extend the reach of genetic medicine to more people, living with more diseases, around the world.

For more information, please visit www.generationbio.com.

Forward-Looking Statements

Any statements in this press release about future expectations, plans and prospects for the company, including statements about the company's strategic plans or objectives, manufacturing plans, cash resources, technology platform, including RES, research and clinical development plans, and preclinical data and other statements containing the words "believes," "anticipates," "plans," "expects," and similar expressions, constitute forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995. Actual results may differ materially from those indicated by such forward-looking statements as a result of various important factors, including: uncertainties inherent in the identification and development of product candidates, including the conduct of research activities, the initiation and completion of preclinical studies and clinical trials and clinical development of the company's product candidates; uncertainties as to the availability and timing of results from preclinical studies and clinical trials; whether results from earlier preclinical studies will be predictive of the results of later preclinical studies and clinical trials; uncertainties regarding the RES manufacturing process; uncertainties regarding the company's ability to assign or sublease its manufacturing property; whether the changes to the company's manufacturing strategy will achieve the anticipated savings; challenges in the manufacture of genetic medicine products; whether the company's cash resources are sufficient to fund the company's operating expenses and capital expenditure requirements for the period anticipated; the impact of the COVID-19 pandemic on the company's business and operations; expectations for regulatory approvals to conduct trials or to market products; as well as the other risks and uncertainties set forth in the "Risk Factors" section of the company's most

recent annual report on Form 10-K and quarterly report on Form 10-Q, which are on file with the Securities and Exchange Commission, and in subsequent filings the company may make with the Securities and Exchange Commission. In addition, the forward-looking statements included in this press release represent the company's views as of the date hereof. The company anticipates that subsequent events and developments will cause the company's views to change. However, while the company may elect to update these forward-looking statements at some point in the future, the company specifically disclaims any obligation to do so. These forward-looking statements should not be relied upon as representing the company's views as of any date subsequent to the date on which they were made.

Investors and Media Contact

CONSOLIDATED BALANCE SHEET DATA

CONSOLIDATED STATEMENTS OF OPERATIONS

(in thousands, except share and per share data)