Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 285

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 285
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 beneficially owned by VGI, the     
 Opportunities GP and the Drawdown GP, respectively. The address of each of these entities and individuals is c/o Viking Global Investors LP, 600 Washington Blvd. Floor 11, Stamford, CT 06901.                                                    |

| (7) | Consists of (i) 547,343 shares of restricted common stock and (ii) 399,902 shares of common stock subject to options 
 exercisable within 60 days of December 31, 2024.                                                                     |

| (8) | Consists of (i) 28,415 shares of restricted common stock and (ii) 112,774 shares of common stock subject to options 
 exercisable within 60 days of December 31, 2024.                                                                    |

| (9) | Consists of (i) 123,537 shares of restricted common stock and (ii) 125,355 shares of common stock subject to options 
 exercisable within 60 days of December 31, 2024.                                                                     |

| (10) | Consists of 89,916 shares of common stock subject to options exercisable within 60 days of December 31, 2024. |

| (11) | Consists of (i) 699,295 shares of restricted common stock and (ii) 727,947 shares of common stock subject to options                     
 exercisable within 60 days of December 31, 2024, held by executive officers and directors, as described in footnotes 7 through 10 above. |

206

DESCRIPTION OF CAPITAL STOCK

General

The following description of our capital stock and
certain provisions of our fifth amended and restated certificate of incorporation and amended and restated bylaws are summaries and are qualified by reference to the fifth amended and restated certificate of incorporation, which will become
effective immediately prior to the completion of this offering, and the amended and restated bylaws, which became effective upon the effectiveness of the registration statement of which this prospectus forms a part. Copies of these documents have
been filed with the SEC as exhibits to our registration statement, of which this prospectus forms a part. The descriptions of the common stock and preferred stock reflect changes to our capital structure that will be in effect on the completion of
this offering.

Upon filing of our fifth amended and restated certificate of incorporation and the completion of this offering, our authorized
capital stock will consist of 500,000,000 shares of common stock, par value