REGENXBIO Announces Pricing of Upsized Public Offering of Common Stock and Pre-funded Warrants ROCKVILLE, Md.

REGENXBIO Announces Pricing of Upsized Public Offering of Common Stock and Pre-funded Warrants

ROCKVILLE, Md., March 6, 2024 /PRNewswire/ REGENXBIO Inc. (Nasdaq: RGNX) a leading clinical-stage biotechnology company seeking to improve lives

through the curative potential of gene therapy, today announced the pricing of an underwritten upsized public offering of 4,565,260 shares of its common stock at the price of $23.00 per share and 1,521,740

pre-funded warrants at a price of $22.9999 per warrant, in each case before underwriting discounts and commissions. The gross proceeds to REGENXBIO from the offering, before deducting the

underwriting discounts and commissions and other offering expenses payable by REGENXBIO, are expected to be approximately $140.0 million. This offering is expected to close on March 11, 2024, subject to customary closing

conditions. In addition, REGENXBIO has granted the underwriters a 30-day option to purchase an additional 913,050 shares of common stock at the public offering price, less underwriting discounts and

Morgan Stanley, Goldman Sachs & Co. LLC, Barclays and Stifel are acting as joint book-running managers of the offering.

The securities described above are being offered by REGENXBIO pursuant to a shelf registration statement on Form S-3

that was filed with the Securities and Exchange Commission (the SEC ) on December 30, 2022 (File No. 333-269086) and became automatically effective upon filing. A preliminary prospectus

supplement relating to and describing the terms of the offering was filed with the SEC and is available on the SEC s website at www.sec.gov. Copies of the preliminary prospectus supplement and the accompanying prospectus relating to this

offering, when available, may be obtained from: Morgan Stanley & Co. LLC, Attention: Prospectus Department, 180 Varick Street, 2nd Floor, New York, NY 10014, or by email at prospectus@morganstanley.com; Goldman Sachs & Co.

LLC, Attention: Prospectus Department, 200 West Street, New York, NY 10282, by phone at (866) 471-2526, or by email at prospectus-ny@ny.email.gs.com;

Barclays Capital Inc., c/o Broadridge Financial Solutions, at 1155 Long Island Avenue, Edgewood, New York 11717 or by email at barclaysprospectus@broadridge.com; or Stifel, Nicolaus & Company, Incorporated, Attention: Prospectus Department,

One Montgomery Street, Suite 3700, San Francisco, CA 94104, by telephone at (415) 364-2720 or by email at syndprospectus@stifel.com. The final terms of the offering will be disclosed in a final prospectus

supplement to be filed with the SEC.

This press release shall not constitute an offer to sell or a solicitation of an offer to buy any of these

securities, nor shall there be any sale of these securities in any state or jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the applicable securities laws of such state or

ABOUT REGENXBIO Inc.

leading clinical-stage biotechnology company seeking to improve lives through the curative potential of gene therapy. Since its founding in 2009, REGENXBIO has pioneered the development of AAV Therapeutics, an innovative class of gene therapy

medicines. REGENXBIO is advancing a pipeline of AAV Therapeutics for retinal and rare diseases, including ABBV-RGX-314 for the treatment of wet AMD and diabetic

retinopathy, being developed in collaboration with AbbVie, RGX-202 for the treatment of Duchenne and RGX-121 for the treatment of MPS II. Thousands of patients have been

treated with REGENXBIO s AAV Therapeutic platform, including Novartis ZOLGENSMA for children with spinal muscular atrophy. Designed to be one-time treatments, AAV Therapeutics have the potential to

change the way healthcare is delivered for millions of people.

FORWARD-LOOKING STATEMENTS

This press release includes forward-looking statements, within the meaning of Section 27A of the Securities Act of 1933, as amended, and

Section 21E of the Securities Exchange Act of 1934, as amended. These statements express a belief, expectation or intention and are generally accompanied by words that convey projected future events or outcomes such as believe,

may, will, estimate, continue, anticipate, assume, design, intend, expect, could, plan, potential,

predict, seek, should, would or by variations of such words or by similar expressions. The forward-looking statements include statements relating to, among other things, statements regarding the timing

and success of the proposed offering and whether REGENXBIO will be able to raise capital through the sale of shares of common stock and pre-funded warrants in the offering. REGENXBIO has based these

forward-looking statements on its current expectations and assumptions and analyses made by REGENXBIO in light of its experience and its perception of historical trends, current conditions and expected future developments, as well as other factors

REGENXBIO believes are appropriate under the circumstances. However, whether actual results and developments will conform with REGENXBIO s expectations and predictions is subject to a number of risks and uncertainties, including the timing of

enrollment, commencement and completion and the success of clinical trials conducted by REGENXBIO, its licensees and its partners, the timing of commencement and completion and the success of preclinical studies conducted by REGENXBIO and its

development partners, the timely development and launch of new products, the ability to obtain and maintain regulatory approval of product candidates, the ability to obtain and maintain intellectual property protection for product candidates and

technology, trends and challenges in the business and markets in which REGENXBIO operates, the size and growth of potential markets for product candidates and the ability to serve those markets, the rate and degree of acceptance of product

candidates, and other factors, many of which are beyond the control of REGENXBIO. Refer to the Risk Factors and Management s Discussion and Analysis of Financial Condition and Results of Operations sections of

REGENXBIO s Annual Report on Form 10-K for the year ended December 31, 2023, and comparable risk factors sections of REGENXBIO s Quarterly Reports on Form 10-Q and other filings, which have been filed with the U.S. Securities and Exchange Commission (SEC) and are available on the SEC s website at www.sec.gov. All of the forward-looking statements made in this

press release are expressly qualified by the cautionary statements contained or referred to herein. The actual results or developments anticipated may not be realized or, even if substantially realized, they may not have the expected consequences to

or effects on REGENXBIO or its businesses or operations. Such statements are not guarantees of future performance and actual results or developments may differ materially from those projected in the forward-looking statements. Readers are cautioned

not to rely too heavily on the forward-looking statements contained in this press release. These forward-looking statements speak only as of the date of this press release. Except as required by law, REGENXBIO does not undertake any obligation, and

specifically declines any obligation, to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.

Zolgensma is a registered trademark of Novartis Gene Therapies. All other trademarks referenced

herein are registered trademarks of REGENXBIO.

Corporate Communications

Chris Brinzey, ICR Westwicke