[144] Sionna Therapeutics, Inc. SEC Filing

Rhea-AI Filing Summary

Sionna Therapeutics, Inc. (SION) Form 144 notice reports a proposed sale of 10,239 common shares through Merrill Lynch on NASDAQ with an aggregate market value of $302,255.28 and an approximate sale date of 09/30/2025. The shares were acquired in a private placement from the issuer on 03/04/2024 for cash. The filing also discloses related secondary sales on 09/29/2025: Atlas Venture Fund XI, LP sold 54,637 shares for $1,644,065.58 and Atlas Venture Opportunity Fund II, L.P. sold 11,468 shares for $345,080.15. The filer certifies no undisclosed material adverse information.

Positive

• Required disclosures provided: acquisition date, nature of acquisition, broker, and sale amounts are specified
• Prior sales disclosed: related institutional sales on 09/29/2025 with gross proceeds documented
• Seller certification: filer affirms no undisclosed material adverse information

Negative

• No information on the identity of the specific individual seller beyond the person for whose account the sale is to be made
• Approximate date only for the proposed sale (09/30/2025) without exact execution details

Insights

Securities Analyst

TL;DR: Insiders/early investors are liquidating a modest position via brokered trades, consistent with post‑private placement selling.

The filing shows a planned brokered sale of 10,239 common shares valued at $302k, acquired in a private placement on 03/04/2024. Two related institutional sales occurred the prior day totaling 66,105 shares and $1.99m in proceeds, indicating coordinated secondary transactions by venture investors. This is routine for holders seeking liquidity after a financing or vesting event. No new financing, litigation, or adverse disclosures are included.

Corporate Governance Expert

TL;DR: Disclosure is limited but satisfies Rule 144 reporting elements for proposed resale.

The notice provides required details: acquisition date, nature (private placement), broker, sale mechanics, and prior three‑month sales by associated funds. The representation regarding absence of material nonpublic information is explicitly made. There is no governance or control change disclosed. From a compliance perspective, the form appears complete for the securities and transactions identified.

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	10239	302255.28	44139823	09/30/2025	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			10239	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 Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/29/2025	11468	345080.15

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 Form 144 disclose about the planned sale?

The form discloses a proposed brokered sale of 10,239 common shares via Merrill Lynch on NASDAQ for an aggregate value of $302,255.28, with an approximate sale date of 09/30/2025.

When were the shares being sold originally acquired according to the filing?

The shares were acquired in a private placement from the issuer on 03/04/2024 and paid for in cash.

Were there any recent sales by related parties disclosed?

Yes. On 09/29/2025, Atlas Venture Fund XI, LP sold 54,637 shares for $1,644,065.58 and Atlas Venture Opportunity Fund II, L.P. sold 11,468 shares for $345,080.15.

Does the filer state any undisclosed material information?

The filer represents by signature that they do not know any material adverse information about the issuer that has not been publicly disclosed.

Through which broker will the proposed sale be executed?

The proposed sale is to be executed through Merrill Lynch, 555 California Street, 18th Floor, San Francisco, CA.