Company: SION
Filing Date: 2025-01-17
Form Type: S-1
Source: 0001193125-25-008474
Chunk: 278

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-01-17
Form: S-1
Chunk 278
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 interest, must be presented to our board of directors or our audit committee for review,
consideration and approval. In approving or rejecting any such proposal, our board of directors or our audit committee is to consider the material facts of the transaction, including whether the transaction is on terms no less favorable than terms
generally available to an unaffiliated third party under the same or similar circumstances and the extent of the related person’s interest in the transaction.

201

PRINCIPAL STOCKHOLDERS The following table sets forth information regarding beneficial ownership of our capital stock as of December 31, 2024 by:

| • |     | each person, or group of affiliated persons, known by us to beneficially own more than 5% of our common stock; |

| • |     | each of our directors; |

| • |     | each of our named executive officers; and |

| • |     | all of our current executive officers and directors as a group. |

We have determined beneficial ownership in accordance with the rules of the SEC. Under these rules, beneficial ownership includes any shares of common stock as to which the individual or entity has sole or shared voting power or investment power. Unless otherwise indicated below, to our knowledge the persons and entities named in the table have sole voting and sole investment power with respect to all shares that they beneficially owned, subject to community property laws where applicable. We have deemed shares of common stock subject to options that are currently exercisable or exercisable within 60 days of December 31, 2024, to be outstanding and to be beneficially owned by the person holding the option for the purpose of computing the percentage ownership of that person but have not treated them as outstanding for the purpose of computing the percentage ownership of any other person. Applicable percentage ownership before the offering is based on an aggregate of 46,679,161 shares of common stock (which includes 110,984 shares of restricted common stock) deemed to be outstanding as of December 31, 2024, after giving effect to the automatic conversion of all outstanding shares of preferred stock into shares of common stock immediately prior to the completion of this offering, and assuming an initial public offering price of $ per share, which is the midpoint of the offering range set forth on the cover page of this prospectus. Applicable percentage ownership after the offering is based on shares of common stock assumed to be outstanding immediately after the completion of this offering (including the sale of shares of common stock in this offering and assuming no exercise of