[144] Sionna Therapeutics, Inc. SEC Filing

Rhea-AI Filing Summary

Sionna Therapeutics, Inc. (SION) filed a Form 144 reporting a proposed sale of 2,047 common shares through Merrill Lynch with an aggregate market value of $61,430.47. The filing lists total outstanding shares as 44,139,823, and an approximate sale date of 10/06/2025. The shares were acquired in a private placement on 03/04/2024 for cash.

The notice also discloses recent related sales by affiliated funds: Atlas Venture Opportunity Fund II and Atlas Venture Fund XI sold multiple blocks between 09/29/2025 and 10/03/2025, including a 54,637-share sale generating $1,644,065.58. The filer certifies no undisclosed material adverse information and includes the standard Rule 10b5-1 and signature attestations.

Positive

• Transaction disclosed under Rule 144 with broker information (Merrill Lynch)
• Shares acquired via private placement on 03/04/2024 were paid in cash

Negative

• Affiliated funds sold sizable blocks between 09/29/2025 and 10/03/2025
• Proposed sale adds to recent supply—2,047 shares on 10/06/2025

Insights

Equity Capital Markets Analyst

TL;DR: A small block of shares is scheduled for sale; affiliated funds previously sold larger blocks.

The filing reports a proposed sale of 2,047 common shares valued at $61,430.47, representing roughly 0.0046% of the stated 44,139,823 outstanding shares.

Prior sales by related Atlas Venture funds between 09/29/2025 and 10/03/2025 show larger liquidity events (for example, 54,637 shares for $1,644,065.58), which could affect free‑float and short‑term supply; monitor trading volume over the next 7–30 days.

Corporate Securities Compliance Specialist

TL;DR: Acquisition and sale timing align with Rule 144 disclosure requirements; the filer affirms no material nonpublic information.

The securities were acquired via a private placement on 03/04/2024 and the proposed sale is being reported under Rule 144, with broker details provided (Merrill Lynch).

The filing includes the required certification regarding knowledge of material adverse information and notes potential reliance on a trading plan or instructions under Rule 10b5‑1; any stated plan date would be the relevant compliance milestone to verify.

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	2047	61430.47	44139823	10/06/2025	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			2047	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 Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/01/2025	35	1048.25
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/03/2025	6548	196451.79
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 Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/03/2025	31198	935996.16

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' (SION) Form 144 disclose?

The Form 144 reports a proposed sale of 2,047 common shares valued at $61,430.47, to be sold via Merrill Lynch around 10/06/2025.

When were the shares for sale acquired by the filer?

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

Did related parties recently sell SION shares?

Yes. Atlas Venture Opportunity Fund II and Atlas Venture Fund XI sold multiple blocks between 09/29/2025 and 10/03/2025, including a 54,637-share sale for $1,644,065.58.

How large is the proposed sale relative to outstanding shares?

The proposed 2,047-share sale is against 44,139,823 shares outstanding, a de minimis proportion of total shares.

Does the filer assert possession of material nonpublic information?

By signing, the filer represents they do not know any undisclosed material adverse information; the form also references potential Rule 10b5‑1 plan adoption if applicable.