Form 4: Atlas Venture Funds Disclose Sionna (SION) Sales Under 10b5-1

Rhea-AI Filing Summary

Director Bruce Booth reported sales of 78,693 shares of Sionna Therapeutics, Inc. (SION) under Rule 10b5-1 plans. The sales occurred on 09/29/2025, 09/30/2025 and 10/01/2025 at weighted-average prices between $29.95 and $30.41, generating dispositions across two Atlas Venture funds. After the reported transactions, Atlas Venture Fund XI, L.P. is shown as beneficially owning 2,821,252 shares and Atlas Venture Opportunity Fund II, L.P. is shown as beneficially owning 794,075 shares. The filing clarifies that the reporting person is a member of the general partner entities and disclaims beneficial ownership except to the extent of any pecuniary interest.

Insights

Corporate Governance Expert

TL;DR: Routine, pre-planned insider sales; transparency maintained through 10b5-1 disclosure.

The filing documents multiple sales executed under established Rule 10b5-1 trading plans for two Atlas Venture funds with Bruce Booth identified as the reporting person and a member of the funds' GP entities. The disclosure includes weighted-average prices and post-transaction beneficial ownership by each fund, and the form contains the required explanatory hierarchy of fund/GP/LLC relationships. From a governance perspective, use of documented 10b5-1 plans and clear ownership structure reduces ambiguity about intent and timing, supporting compliance with insider trading rules.

Securities Analyst

TL;DR: Sales are sizable in share count but leave the funds with multi-million and mid-six-figure holdings; impact appears informational only.

The combined disposition totals 78,693 shares sold across three dates with reported weighted-average prices in the $29.95–$30.41 range. Post-sale beneficial ownership remains significant: Atlas Venture Fund XI holds 2,821,252 shares and Atlas Venture Opportunity Fund II holds 794,075 shares. There is no indication in the Form 4 of derivative transactions or amendments affecting economic exposure beyond the disclosed sales. Absent additional company-level disclosures, these reported sales are a liquidity event by investors rather than a corporate action affecting operations or financial guidance.

SEC Form 4

FORM 4

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940

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Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

1. Name and Address of Reporting Person^*

Booth Bruce

(Last) (First) (Middle)

C/O SIONNA THERAPEUTICS, INC.

21 HICKORY DRIVE, SUITE 500

(Street)

WALTHAM MA 02451

(City) (State) (Zip)

2. Issuer Name and Ticker or Trading Symbol
Sionna Therapeutics, Inc. [ SION ]

5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

X	Director		10% Owner
	Officer (give title below)		Other (specify below)

3. Date of Earliest Transaction (Month/Day/Year)
09/29/2025

4. If Amendment, Date of Original Filed (Month/Day/Year)

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	3. Transaction Code (Instr. 8)		4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	09/29/2025		S(1)		54,637	D	$30.09(2)	2,831,656	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	09/29/2025		S(4)		11,468	D	$30.09(2)	796,259	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	09/30/2025		S(1)		10,239	D	$30.06(6)	2,821,417	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	09/30/2025		S(4)		2,149	D	$30.06(6)	794,110	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/01/2025		S(1)		165	D	$29.95	2,821,252	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/01/2025		S(4)		35	D	$29.95	794,075	I	By Atlas Venture Opportunity Fund II, L.P.(5)

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares

Explanation of Responses:

1. This transaction was made pursuant to a Rule 10b5-1 trading plan adopted by Atlas Venture Fund XI, L.P. on February 25, 2025.

2. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $29.95 to $30.41 inclusive. The Reporting Persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (2) and (6).

3. These shares are held directly by Atlas Venture Fund XI, L.P. ("AVF XI "). The general partner of AVF XI is Atlas Venture Associates XI, L.P. ("AVA XI LP"). Atlas Venture Associates XI, LLC ("AVA XI LLC") is the general partner of AVA XI LP. The Reporting Person is a member of AVA XI LLC and disclaims beneficial ownership of such securities held by AVF XI, except to the extent of his pecuniary interest therein, if any.

4. This transaction was made pursuant to a Rule 10b5-1 trading plan adopted by Atlas Venture Opportunity Fund II, L.P. on February 25, 2025.

5. These shares are held directly by Atlas Venture Opportunity Fund II, L.P. ("AVOF II"). The general partner of AVOF II is Atlas Venture Associates Opportunity II, L.P. ("AVAO II LP"). Atlas Venture Associates Opportunity II, LLC ("AVAO II LLC") is the general partner of AVAO II LP. The Reporting Person is a member of AVAO II LLC and disclaims beneficial ownership of such securities held by AVOF II, except to the extent of his pecuniary interest therein, if any.

6. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $29.95 to $30.20 inclusive.

	/s/ Ommer Chohan, Attorney-in-Fact	10/01/2025
	** Signature of Reporting Person	Date
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