SION Form 144 Discloses 35 Shares Sale; Atlas Venture Funds Sold Large Blocks

Rhea-AI Filing Summary

Sionna Therapeutics (SION) filed a Form 144 reporting a proposed sale of 35 common shares through Merrill Lynch with an approximate sale date of 10/01/2025. The shares were acquired in a private placement on 03/04/2024 for cash. The filing lists 44,139,823 shares outstanding and an aggregate market value for the proposed sale of 1,029.35. The filing also discloses recent sales by affiliated funds: Atlas Venture Opportunity Fund II and Atlas Venture Fund XI executed multiple common-stock sales on 09/29/2025 and 09/30/2025, including transactions of 11,468, 2,149, 54,637, and 10,239 shares with gross proceeds reported for each sale. The notice includes the required representation that the seller is unaware of undisclosed material adverse information.

Positive

• Form 144 disclosure is complete with acquisition, broker, sale date, and recent sales disclosed
• Seller includes required representation that no undisclosed material adverse information is known

Negative

• Multiple sizeable sales by Atlas Venture funds on 09/29/2025 and 09/30/2025 (totaling 79,493 shares reported across four trades) could increase available supply
• Aggregate market value reported for the proposed 35-share sale is small relative to outstanding shares, suggesting the single notice is immaterial but larger fund sales may be material

Insights

Securities Analyst

TL;DR Routine insider/affiliate selling with clear disclosures; recent larger fund sales are more material than the single proposed 35-share sale.

The Form 144 reports a proposed sale of 35 shares acquired via private placement in March 2024 and scheduled for October 1, 2025, routed through Merrill Lynch. The filing also documents multiple sizeable block sales by Atlas Venture funds on September 29 and 30, 2025, with explicit gross proceeds for each trade. From an investor perspective, the single 35-share notice is immaterial relative to the issuer's ~44.14 million shares outstanding, while the fund sales appear more consequential and merit monitoring for potential impact on float and short-term supply.

Corporate Governance Expert

TL;DR Disclosure meets Rule 144 requirements; representation on material nonpublic information is included.

The filing contains the standard Rule 144 fields: acquisition details, nature of acquisition (private placement), broker information, proposed sale date, and recent three-month sales by related parties. It includes the seller's signed representation regarding nonpublic material information. Procedurally the document appears complete for a Form 144, showing compliance with disclosure obligations; however, governance implications depend on the context of the larger fund sales disclosed.

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	35	1029.35	44139823	10/01/2025	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			35	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 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 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

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 the Form 144 for SION report?

The Form 144 reports a proposed sale of 35 common shares via Merrill Lynch with an approximate sale date of 10/01/2025, and recent sales by Atlas Venture funds on 09/29/2025 and 09/30/2025.

When were the 35 shares acquired and how?

The 35 shares were acquired on 03/04/2024 in a private placement and paid for in cash.

How many shares does Sionna Therapeutics have outstanding per the filing?

The filing shows 44,139,823 shares outstanding.

What recent sales did Atlas Venture report in the filing?

Atlas Venture Opportunity Fund II sold 11,468 and 2,149 shares on 09/29/2025 and 09/30/2025 respectively; Atlas Venture Fund XI sold 54,637 and 10,239 shares on those same dates, with gross proceeds reported for each trade.

Does the filing state whether the seller knows of any undisclosed material information?

Yes. The notice includes the seller's representation that they do not know any material adverse information not publicly disclosed.