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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	OMB APPROVAL OMB Number: 3235-0287 Estimated average burden hours per response: 0.5
X	Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
	Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

1. Name and Address of Reporting Person* Atlas Venture Fund XI, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (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) Director X 10% Owner Officer (give title below) Other (specify below)
	3. Date of Earliest Transaction (Month/Day/Year) 02/10/2025
	4. If Amendment, Date of Original Filed (Month/Day/Year)	6. Individual or Joint/Group Filing (Check Applicable Line) Form filed by One Reporting Person X 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	02/10/2025		C		2,886,293	A	(1)	2,886,293	D(2)
Common Stock	02/10/2025		C		747,727	A	(1)	747,727	I	By Atlas Venture Opportunity Fund II, L.P.(3)
Common Stock	02/10/2025		P		60,000	A	$18	807,727	I	By Atlas Venture Opportunity Fund II, L.P.(3)

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
Series Seed convertible preferred stock	(1)	02/10/2025		C			1,051,051	(1)	(1)	Common Stock	719,355	$0	0	D(2)
Series A convertible preferred stock	(1)	02/10/2025		C			1,806,317	(1)	(1)	Common Stock	1,236,271	$0	0	D(2)
Series B convertible preferred stock	(1)	02/10/2025		C			1,298,332	(1)	(1)	Common Stock	888,599	$0	0	D(2)
Series C convertible preferred stock	(1)	02/10/2025		C			61,463	(1)	(1)	Common Stock	42,068	$0	0	D(2)
Series C convertible preferred stock	(1)	02/10/2025		C			1,092,504	(1)	(1)	Common Stock	747,727	$0	0	I	By Atlas Venture Opportunity Fund II, L.P.(3)

1. Name and Address of Reporting Person* Atlas Venture Fund XI, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip) Relationship of Reporting Person(s) to Issuer Director X 10% Owner Officer (give title below) Other (specify below)

1. Name and Address of Reporting Person* Atlas Venture Associates XI, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip) Relationship of Reporting Person(s) to Issuer Director X 10% Owner Officer (give title below) Other (specify below)

1. Name and Address of Reporting Person* Atlas Venture Associates XI, LLC (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip) Relationship of Reporting Person(s) to Issuer Director X 10% Owner Officer (give title below) Other (specify below)

1. Name and Address of Reporting Person* Atlas Venture Opportunity Fund II, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip) Relationship of Reporting Person(s) to Issuer Director X 10% Owner Officer (give title below) Other (specify below)

1. Name and Address of Reporting Person* Atlas Venture Associates Opportunity II, LP (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip) Relationship of Reporting Person(s) to Issuer Director X 10% Owner Officer (give title below) Other (specify below)

1. Name and Address of Reporting Person* Atlas Venture Associates Opportunity II, LLC (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip) Relationship of Reporting Person(s) to Issuer Director X 10% Owner Officer (give title below) Other (specify below)

Explanation of Responses:

1. Each share of Series Seed convertible preferred stock, Series A convertible preferred stock, Series B convertible preferred stock and Series C convertible preferred stock (collectively, the "Preferred Stock") automatically converted into shares of the Issuer's Common Stock on a one-for-1.4611 basis upon the closing of the Issuer's initial public offering on February 10, 2025. The Preferred Stock had no expiration date.

2. 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. Each of AVA XI LP and AVA XI LLC disclaims Section 16 beneficial ownership of the securities held by AVF XI, except to the extent of its pecuniary interest therein, if any.

3. 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. Each of AVAO II LP and AVAO II LLC disclaims Section 16 beneficial ownership of the securities held by AVOF II, except to the extent of its pecuniary interest therein, if any.

Remarks:

4. Reflects shares purchased in the Issuer's initial public offering.

	Atlas Venture Fund XI, L.P., By: Atlas Venture Associates XI, L.P., its general partner, by Atlas Venture Associates XI, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer	02/10/2025
	Atlas Venture Associates XI, L.P., by Atlas Venture Associates XI, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer	02/10/2025
	Atlas Venture Associates XI, LLC, By: /s/ Ommer Chohan, Chief Financial Officer	02/10/2025
	Atlas Venture Opportunity Fund II, L.P., By: Atlas Venture Associates Opportunity II, L.P., its General Partner, By: Atlas Venture Associates Opportunity II, LLC, its General Partner, By: /s/ Ommer Chohan, Chief Financial Officer	02/10/2025
	Atlas Venture Associates Opportunity II, L.P., By: Atlas Venture Associates Opportunity II, LLC, its General Partner, By: /s/ Ommer Chohan, Chief Financial Officer	02/10/2025
	Atlas Venture Associates Opportunity II, LLC, By: /s/ Ommer Chohan, Chief Financial Officer	02/10/2025
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 4 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
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