SION Form 144 Filed for 165-Share Sale; Atlas Venture Dispositions Listed

Rhea-AI Filing Summary

Sionna Therapeutics filed a Form 144 reporting a proposed sale of 165 common shares acquired in a private placement on 03/04/2024. The sale is to be executed through Merrill Lynch (San Francisco) for an approximate aggregate market value of $4,852.65 with an approximate sale date of 10/01/2025. The company notes 44,139,823 shares outstanding. Related recent dispositions by affiliated funds include four sales on 09/29/2025–09/30/2025 totaling 78,493 shares for aggregate gross proceeds of $2,389,102.99.

Positive

• Clear disclosure of acquisition date, nature (private placement), broker, and proposed sale date
• Recent related-party sales are fully itemized with dates, amounts, and gross proceeds

Negative

• Insufficiently detailed signature/notice date text in the provided excerpt (signature and date fields appear blank)
• Affiliated fund sales totaled 78,493 shares for ~$2.39M, which may prompt investor questions despite being small relative to outstanding shares

Insights

Securities Analyst

TL;DR: Small planned sale and recent fund dispositions are disclosed; amounts are immaterial relative to outstanding shares.

The filing reports a modest proposed sale of 165 shares acquired in a private placement and lists recent secondary sales by Atlas Venture entities totaling 78,493 shares and roughly $2.39 million in proceeds over two days. Given the issuer's 44.1 million shares outstanding, these transactions represent a small percentage of the float and are unlikely to materially affect valuation. The disclosure is routine under Rule 144.

Compliance/Risk Analyst

TL;DR: Documentation appears compliant with Rule 144 requirements; signatures and dates must be present on final filing.

The form specifies acquisition details, broker information, and recent sales by related funds, which aligns with Rule 144 disclosure elements. The filer affirms absence of undisclosed material information. The filing should be reviewed to confirm the signature block and exact notice date are present in the official submission to ensure full compliance.

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	165	4852.65	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			165	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 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

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 Sionna Therapeutics' Form 144 report?

The Form 144 reports a proposed sale of 165 common shares acquired in a private placement on 03/04/2024, to be sold via Merrill Lynch with an approximate sale date of 10/01/2025.

How much is the proposed sale worth on the Form 144 for SION?

The filing lists an aggregate market value of $4,852.65 for the 165 shares proposed to be sold.

What recent sales by related parties are disclosed in the filing?

Atlas Venture Fund XI, LP and Atlas Venture Opportunity Fund II, L.P. sold a combined 78,493 shares on 09/29/2025–09/30/2025 for total gross proceeds of $2,389,102.99.

Through which broker will the SION shares be sold?

The broker named in the filing is Merrill Lynch, 555 California Street, 18th Floor, San Francisco, CA.

How many shares outstanding does the filing state for Sionna Therapeutics?

The filing reports 44,139,823 shares outstanding.