Company: SION
Filing Date: 2025-02-03
Form Type: S-1/A
Source: 0001193125-25-018825
Chunk: 280

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-03
Form: S-1/A
Chunk 280
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| Entities affiliated with Atlas Venture(2) |     |                    | 1,298,332 |     | $                    | 12,674,317 |
| OrbiMed Private Investments VIII, LP(3)   |     |                    | 2,560,951 |     | $                    | 25,000,004 |
| Entities affiliated with RA Capital(4)    |     |                    | 3,079,242 |     | $                    | 30,059,560 |
| Entities affiliated with TPG Growth(5)    |     |                    | 2,275,528 |     | $                    | 22,213,704 |

| (1) | Additional details regarding these stockholders and their equity holdings are included in this prospectus under the 
 section “Principal Stockholders.”                                                                                   |

| (2) | Consists of 1,298,332 shares of Series B preferred stock held by Atlas Venture Fund XI, L.P. Atlas Venture Fund XI, L.P.                                                                                                                   
 is an affiliate of Atlas Venture. Entities affiliated with Atlas Ventures beneficially own more than 5% of our outstanding capital stock. Bruce Booth, D.Phil., a member of our board of directors, is a general partner at Atlas Venture. |

| (3) | OrbiMed Private Investments VIII, LP (“OrbiMed”) beneficially owns more than 5% of our outstanding capital 
 stock. Peter A. Thompson, M.D., a member of our board of directors, is a member at OrbiMed.                |

| (4) | Consists of (i) 2,155,469 shares of Series B preferred stock held by RA Capital Nexus Fund, L.P. and (ii) 923,773 shares                                                                                                                               
 of Series B preferred stock held by RA Capital Healthcare Fund, L.P. Entities affiliated with RA Capital beneficially own more than 5% of our outstanding capital stock. Joshua Resnick, M.D., M.B.A., a member of our board of directors, is a senior 
 managing director at RA Capital.                                                                                                                                                                                                                       |

| (5) | Consists of 2,275,528 shares of Series B preferred stock held by The Rise Fund Sling, L.P. Entities affiliated with TPG                                                   
 Growth beneficially owns more than 5% of our outstanding capital stock. Lucian Iancovici, M.D., a member of