Company: RGNX
Filing Date: 2025-03-13
Form Type: POSASR
Source: 0001193125-25-053917
Chunk: 1

Company: REGENXBIO Inc.
Filing Date: 2025-03-13
Form: POSASR
Chunk 1
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 Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐ If this form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box. ☒ If this form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. ☐ Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-acceleratedfiler, a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2under the Exchange Act.

| Large accelerated filer |     | ☐                                               |     | Accelerated filer         |     | ☒ |
| Non-accelerated filer   |     | ☐ (Do not check if a smaller reporting company) |     | Smaller reporting company |     | ☐ |
|                         |     |                                                 |     | Emerging growth company   |     | ☐ |

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ☐

E XPLANATORYN OTE

This Post-Effective Amendment No. 1 to the Registration Statement on Form S-3 (File No. 333-269086) (the “Registration Statement”) of REGENXBIO Inc. is being filed because we will no longer be a well-known seasoned issuer, as such term is defined in Rule 405 under the Securities Act
of 1933, upon the filing of our Annual Report Form 10-K for the year ended December 31, 2024 because the worldwide market value of our outstanding common stock held by
non-affiliates was less than $700 million during the 60-day period preceding the date of such filing. Pursuant to interpretive guidance published by the United
States Securities and Exchange Commission’s Division of Corporate Finance, this Post-Effective Amendment No. 1 is being filed to add disclosure to the Registration