REGENXBIO Inc美股招股说明书(2026-07-16版)

Common Stock We are offeringshares of our common stock with an aggregate public offering price of approximately $pursuant to this prospectus supplement. Our common stock is listed for trading on The Nasdaq Global Select Market(“Nasdaq”) under the symbol “RGNX.” On July15, 2026, the closing sales price of our common stock on Nasdaq was$12.27 per share. We are a “smaller reporting company” under applicable Securities and Exchange Commission (“SEC”) rules and, as such, haveelected to comply with certain reduced public company disclosure requirements in this prospectus supplement and future filingsthat we make with the SEC. Investing in our common stock involves significant risks. Please carefully consider the risksdiscussed in “Risk Factors” beginning on page S-7 of this prospectus supplement and in our filingswith the SEC that are incorporated by reference into this prospectus supplement before making adecision to invest in our common stock. PerShareTotalPublic offering price$$Underwriting discounts and commissions(1)$$Proceeds, before expenses, to us$$ (1)We have agreed to reimburse the underwriters for certain FINRA-related offering expenses. See “Underwriting.” We have granted the underwriters an option to purchase up to an additionalshares of our common stock with anaggregate public offering price of approximately $at the public offering price, less the underwriting discount, within 30days from the date of this prospectus supplement. Neither the SEC nor any state securities commission has approved or disapproved of these securities or passed upon theaccuracy or adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contraryis a criminal offense. The information in this preliminary prospectus supplement and the accompanying prospectus is not complete and may be changed. This preliminary prospectussupplement and the accompanying prospectus are not an offer to sell these securities and are not soliciting an offer to buy these securities in any jurisdiction where theoffer or sale is not permitted.The underwriters expect to deliver the shares of common stock against payment therefor on or about July, 2026. Mizuho The date of this prospectus supplement is July, 2026. Table of Contents TABLE OF CONTENTS Prospectus Supplement About this Prospectus SupplementSpecial Note Regarding Forward-Looking StatementsProspectus Supplement SummaryThe OfferingRisk FactorsUse of ProceedsDilutionMaterial U.S. Federal Income Tax Considerations for Non-U.S. Holders of Common StockUnderwritingLegal MattersExpertsWhere You Can Find More InformationIncorporation of Documents By Reference Prospectus Special Note Regarding Forward-Looking StatementsAbout this ProspectusWhere You Can Find More InformationThe CompanyRisk FactorsUse of ProceedsDescription of Capital StockDescription of WarrantsDescription of Debt SecuritiesDescription of RightsDescription of UnitsPlan of DistributionLegal MattersExperts Table of Contents ABOUT THIS PROSPECTUS SUPPLEMENT This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stockand updates the information contained in the accompanying prospectus and the documents incorporated by reference herein and therein. The second partis the accompanying prospectus, which provides more general information about securities that we may offer from time to time, some of which does notapply to this offering. To the extent the information contained in this prospectus supplement differs or varies from the information contained in theaccompanying prospectus or documents previously filed with the SEC that are incorporated by reference herein, the information in this prospectussupplement will supersede such information. For a more detailed understanding of an investment in our common stock, you should read both thisprospectus supplement and the accompanying prospectus, together with additional information described under the heading “Where You Can Find MoreInformation.” This prospectus supplement is part of a shelf registration statement on Form S-3 (File No.333-291816) that we filed with the SEC onNovember26, 2025 and that became effective on December12, 2025. Under the shelf registration process, we may from time to time offer and sell anycombination of the securities described in the accompanying prospectus in one or more offerings. Neither we nor the underwriters have authorized anyone to provide you with information that is different from or in addition to thatcontained or incorporated by reference in this prospectus supplement, the accompanying prospectus or any free writing prospectus preparedby us or on our behalf. Neither we nor the underwriters take any responsibility for, and can provide no assurance as to the reliability of, anyinformation that others may provide. Neither we nor the underwriters are making an offer to sell or soliciting an offer to buy shares of ourcomm