Company: SION
Filing Date: 2025-02-03
Form Type: S-1/A
Source: 0001193125-25-018825
Chunk: 284

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-03
Form: S-1/A
Chunk 284
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, which will require us to indemnify them. For more information regarding these agreements, see the section titled “Management—Limitations on Liability and Indemnification.”

Policies and Procedures for Transactions with Related Persons

In connection with this offering, we adopted, effective upon the effectiveness of the registration statement of which this prospectus forms a part, a
written policy that our executive officers, directors, nominees for election as a director, beneficial owners of more than 5% of any series of our common stock and any members of the immediate family of any of the foregoing persons are not permitted
to enter into a related person transaction with us without the approval or ratification of our board of directors or our audit committee. Any request for us to enter into a transaction with an executive officer,

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director, nominee for election as a director, beneficial owner of more than 5% of any series of our common stock or any member of the immediate family of any of the foregoing persons, in which
the amount involved exceeds $120,000 (or, if less, 1% of the average of our total assets in a fiscal year) and such person would have a direct or indirect 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.

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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