[144] Sionna Therapeutics, Inc. SEC Filing

Rhea-AI Filing Summary

Sionna Therapeutics filed a Form 144 notifying a proposed sale of 31,198 common shares acquired in a private placement on 03/04/2024. The filing reports an aggregate market value of $914,101.40 and lists the approximate sale date as 10/03/2025, with Merrill Lynch named as the broker and the securities listed on NASDAQ. The company shows 44,139,823 shares outstanding. The filing also discloses recent sales by related funds in late September–early October 2025 totaling multiple transactions (largest: 54,637 shares for $1,644,065.58 on 09/29/2025).

Form 144 Filer Information	UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Form 144 NOTICE OF PROPOSED SALE OF SECURITIES PURSUANT TO RULE 144 UNDER THE SECURITIES ACT OF 1933
FORM 144

144: Filer Information

Filer CIK
Filer CCC
Is this a LIVE or TEST Filing?	LIVE TEST
Submission Contact Information
Name
Phone
E-Mail Address

144: Issuer Information

Name of Issuer

SEC File Number

Address of Issuer

Phone

Name of Person for Whose Account the Securities are To Be Sold

See the definition of "person" in paragraph (a) of Rule 144. Information is to be given not only as to the person for whose account the securities are to be sold but also as to all other persons included in that definition. In addition, information shall be given as to sales by all persons whose sales are required by paragraph (e) of Rule 144 to be aggregated with sales for the account of the person filing this notice.

Relationship to Issuer

144: Securities Information

Title of the Class of Securities To Be Sold	Name and Address of the Broker	Number of Shares or Other Units To Be Sold	Aggregate Market Value	Number of Shares or Other Units Outstanding	Approximate Date of Sale	Name the Securities Exchange
Common	Merrill Lynch 555 California Street 18th Floor San Francisco CA 94104	31198	914101.4	44139823	10/03/2025	NASDAQ

Furnish the following information with respect to the acquisition of the securities to be sold and with respect to the payment of all or any part of the purchase price or other consideration therefor:

144: Securities To Be Sold

Title of the Class	Date you Acquired	Nature of Acquisition Transaction	Name of Person from Whom Acquired	Is this a Gift?	Date Donor Acquired	Amount of Securities Acquired	Date of Payment	Nature of Payment *
Common	03/04/2024	Private Placement	Issuer			31198	03/04/2024	Cash

* If the securities were purchased and full payment therefor was not made in cash at the time of purchase, explain in the table or in a note thereto the nature of the consideration given. If the consideration consisted of any note or other obligation, or if payment was made in installments describe the arrangement and state when the note or other obligation was discharged in full or the last installment paid.

Furnish the following information as to all securities of the issuer sold during the past 3 months by the person for whose account the securities are to be sold.

144: Securities Sold During The Past 3 Months

Name and Address of Seller	Title of Securities Sold	Date of Sale	Amount of Securities Sold	Gross Proceeds
Atlas Venture Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/29/2025	54637	1644065.58
Atlas Venture Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/30/2025	10239	307762.84
Atlas Venture Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/01/2025	165	4941.75
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/29/2025	11468	345080.15
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/30/2025	2149	64594.43
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/01/2025	35	1048.25

144: Remarks and Signature

Remarks

Date of Notice

Date of Plan Adoption or Giving of Instruction, If Relying on Rule 10b5-1

ATTENTION:

The person for whose account the securities to which this notice relates are to be sold hereby represents by signing this notice that he does not know any material adverse information in regard to the current and prospective operations of the Issuer of the securities to be sold which has not been publicly disclosed. If such person has adopted a written trading plan or given trading instructions to satisfy Rule 10b5-1 under the Exchange Act, by signing the form and indicating the date that the plan was adopted or the instruction given, that person makes such representation as of the plan adoption or instruction date.

Signature

ATTENTION: Intentional misstatements or omission of facts constitute Federal Criminal Violations (See 18 U.S.C. 1001)

FAQ

What does SION's Form 144 disclose about the proposed sale?

The Form 144 notifies a proposed sale of 31,198 common shares with an aggregate market value of $914,101.40, approx sale date 10/03/2025, via Merrill Lynch on NASDAQ.

When and how were the shares being sold acquired?

The shares were acquired on 03/04/2024 in a private placement from the issuer, with payment made in cash on 03/04/2024.

How many shares of SION are outstanding per the filing?

The filing lists 44,139,823 shares outstanding.

Were there any recent related-party sales disclosed?

Yes. Related funds reported multiple sales on 09/29/2025, 09/30/2025, and 10/01/2025, including a sale of 54,637 shares for $1,644,065.58 on 09/29/2025.

Who is the broker handling the proposed sale?

The broker named in the Form 144 is Merrill Lynch, located at 555 California Street, San Francisco, CA.