Company: RGNX
Filing Date: 2025-06-13
Form Type: S-3
Source: 0001193125-25-140870
Chunk: 27

Company: REGENXBIO Inc.
Filing Date: 2025-06-13
Form: S-3
Chunk 27
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 such director, officer or controlling person in connection with the securities being registered, the
registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question of whether such indemnification by it is against public policy as expressed in the
Securities Act of 1933 and will be governed by the final adjudication of such issue.

II-5

SIGNATURES Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Rockville, State of Maryland, on the 13th day of June, 2025.

| REGENXBIO INC. |     |                                       |
| By:            |     | /s/ Curran Simpson                    |
|                |     | Curran Simpson                        |
|                |     | President and Chief Executive Officer |

POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints Curran Simpson, Mitchell Chan and Patrick J. Christmas, and each of them, as his true and lawful attorneys-in-factand agents, with full power of substitution and resubstitution, for him and in his name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this registration statement, and to file the same, with all exhibits thereto, and other documents in connection therewith, with the United States Securities and Exchange Commission, granting unto said attorneys-in-factand agents, and each of them, full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith and about the premises, as fully to all intents and purposes as he might or could do in person, hereby ratifying and confirming all that said attorneys-in-factand agents, or any of them, or their or his substitute or substitutes, may lawfully do or cause to be done by virtue hereof. Pursuant to the requirements of the Securities Act of 1933, this registration statement has been signed by the following persons in the capacities indicated on June 13, 2025.

| Signature                |     | Title                                                          |
| /s/ Curran Simpson       
 Curran Simpson