[144] Sionna Therapeutics, Inc. SEC Filing

Rhea-AI Filing Summary

Sionna Therapeutics, Inc. submitted a Form 144 reporting a proposed sale of 3,511 common shares with an aggregate market value of $99,536.85, representing part of the holder's holding against 44,139,823 shares outstanding. The filing names Merrill Lynch as the broker and lists an approximate sale date of 10/10/2025. The securities were acquired in a private placement on 03/04/2024 and paid for in cash.

The filing also discloses multiple securities sales by affiliated funds over the prior three months: Atlas Venture Opportunity Fund II and Atlas Venture Fund XI executed several transactions between 09/29/2025 and 10/06/2025, with individual gross proceeds ranging from $1,048.25 to $1,644,065.58. The filer certifies no undisclosed material adverse information and includes the standard Rule 144 attestation language.

Positive

• Proper disclosure of a Rule 144 sale with broker identified (Merrill Lynch) and acquisition details provided
• Attestation that the seller does not possess undisclosed material adverse information

Negative

• Active recent sales by affiliated funds between 09/29/2025 and 10/06/2025 could increase near-term supply
• Single-entry large disposals (e.g., $1,644,065.58) may pressure liquidity depending on market demand

Insights

Securities Counsel

Form 144 documents a routine Rule 144 notice for an exempt-holder sale and lists recent affiliated fund dispositions.

The notice shows a proposed resale under Rule 144 of 3,511 shares acquired in a private placement on 03/04/2024, with $99,536.85 market value and a broker named as Merrill Lynch. The filer affirms compliance statements required by federal law.

Risks hinge on timing and aggregation rules: recent open-market sales by affiliated funds between 09/29/2025 and 10/06/2025 could affect available volume under Rule 144's aggregation and holding-period calculations; monitor whether the aggregated sales remain within safe-harbor limits over the next weeks.

Venture Capital Analyst

Affiliated venture funds have been actively monetizing positions via multiple sales in late September–early October 2025.

Transactions by Atlas Venture Opportunity Fund II and Atlas Venture Fund XI list material lot sizes (e.g., 54,637 and 31,198 in single entries) with gross proceeds up to $1,644,065.58, indicating staged liquidity events by early investors.

For investors, watch cumulative volumes versus outstanding float (44,139,823) and upcoming filing activity in the next 30 days to gauge continued selling pressure and potential impact on short-term trading liquidity.

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	3511	99536.85	44139823	10/10/2025	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			3511	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 Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/06/2025	2047	61430.47
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
Atlas Venture Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/06/2025	9753	292687.53

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 Form 144 filed by SION mean for investors?

It notifies the market of a proposed sale of 3,511 common shares valued at $99,536.85 with an approximate sale date of 10/10/2025 under Rule 144.

Who is selling SION shares according to the filing?

The filing shows sales by affiliated entities including Atlas Venture Opportunity Fund II and Atlas Venture Fund XI, plus a proposed sale by the filer via Merrill Lynch.

When were the shares to be sold originally acquired?

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

How much of Sionna's outstanding stock does the proposed sale represent?

The proposed sale of 3,511 shares relates to an issuer with 44,139,823 shares outstanding, representing a de minimis fraction of total shares.

What recent selling activity is disclosed in the filing?

Multiple sales from 09/29/2025 to 10/06/2025 list combined lots (e.g., 54,637, 31,198) with gross proceeds up to $1,644,065.58.