SION Rule 144 Notice: 2,149 Shares Proposed Sale; Prior Day Sales Disclosed

Rhea-AI Filing Summary

Sionna Therapeutics notice reports a proposed sale under Rule 144 of 2,149 common shares through Merrill Lynch (San Francisco) with an approximate aggregate market value of $63,438.48 and an intended sale date of 09/30/2025 on NASDAQ. The filing shows the shares were acquired on 03/04/2024 in a private placement from the issuer, paid in cash. The filing also discloses related sales in the prior three months: 11,468 shares (gross proceeds $345,080.15) by Atlas Venture Opportunity Fund II, L.P. and 54,637 shares (gross proceeds $1,644,065.58) by Atlas Venture Fund XI, LP on 09/29/2025.

Insights

Securities Analyst

TL;DR: Notice of proposed sale under Rule 144; small nominated block and larger recent disposals by affiliated funds.

The filing documents a proposed sale of 2,149 common shares purchased in a private placement on 03/04/2024 and scheduled for sale on 09/30/2025 via Merrill Lynch on NASDAQ for an aggregate value of $63,438.48. It also discloses substantial related sales the prior day: 11,468 and 54,637 shares generating ~$345k and ~$1.64m respectively. From a securities compliance view, the form meets Rule 144 disclosure by stating acquisition date, broker, planned sale date, and recent dispositions. The information is procedural and does not include company operational or financial metrics.

Corporate Governance Expert

TL;DR: Routine insider/affiliate disposition filings showing holdings monetization, with full transactional detail provided.

The notice indicates monetization of founder/affiliate-related holdings acquired in a private placement and subsequent large disposals by two Atlas Venture funds the day before. The filing includes necessary representations about absence of undisclosed material information and references possible reliance on trading plans. This is a standard compliance disclosure; it documents transfer activity but contains no governance changes, remuneration details, or material corporate actions.

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	2149	63438.48	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			2149	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 Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/29/2025	54637	1644065.58

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 2,149 common shares, aggregate market value $63,438.48, planned for sale on 09/30/2025 through Merrill Lynch on NASDAQ.

When and how were the shares being sold acquired?

The 2,149 shares were acquired on 03/04/2024 in a private placement from the issuer and paid in cash.

Were there any other recent sales disclosed in the filing?

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

Which broker is handling the proposed sale?

Merrill Lynch, 555 California Street, 18th Floor, San Francisco, CA 94104, is listed as the broker.

Does the filing state any material undisclosed information about the company?

The filer's signature box includes the representation that they do not know any material adverse information about the issuer that has not been publicly disclosed; no specific undisclosed material facts are stated in this filing.