Company: RGNX
Filing Date: 2025-03-13
Form Type: POS AM
Source: 0001193125-25-053998
Chunk: 1

Company: REGENXBIO Inc.
Filing Date: 2025-03-13
Form: POS AM
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. 2 to the Registration Statement on Form S-3 (File No. 333-269086), as previously amended by Post-Effective Amendment No. 1 (as so amended, the “Registration Statement”) of REGENXBIO Inc. is being filed for the purpose of updating the
Registration Statement to reflect the fact that we are no longer a well-known seasoned issuer, as such term is defined in Rule 405 under the Securities Act of 1933 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 we filed our Annual Report on Form 10-K for
the year ended December 31, 2024. Pursuant to interpretive guidance published by the United States Securities and Exchange Commission’s