Sigilon Therapeutics (SGTX) SC 13G/A Sigilon Therapeutics / Flagship Ventures Fund V ownership change

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

Sigilon Therapeutics, Inc.

(Name of Issuer)

Common Stock, $0.001 par value per share

(Title of Class of Securities)

82657L 10 7

(CUSIP Number)

December 31, 2023

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☐ Rule 13d-1(c)

☒ Rule 13d-1(d)

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 82657L 10 7 Page 2 of 11 Pages

1	NAMES OF REPORTING PERSONS  I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)  Flagship Ventures Fund V, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)  (a) ☐  (b) ☒
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION  Delaware

NUMBER OF SHARES  BENEFICIALLY  OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER  0
	6	SHARED VOTING POWER  0
	7	SOLE DISPOSITIVE POWER  0
	8	SHARED DISPOSITIVE POWER  0

9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON  0
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)  ☐
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9  0.0%
12	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)  PN

CUSIP No. 82657L 10 7 Page 3 of 11 Pages

1	NAMES OF REPORTING PERSONS  I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)  Flagship Ventures Fund V General Partner LLC
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)  (a) ☐  (b) ☒
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION  Delaware

NUMBER OF SHARES  BENEFICIALLY  OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER  0
	6	SHARED VOTING POWER  0
	7	SOLE DISPOSITIVE POWER  0
	8	SHARED DISPOSITIVE POWER  0

9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON  0
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)  ☐
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9  0.0%
12	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)  OO

CUSIP No. 82657L 10 7 Page 4 of 11 Pages

1	NAMES OF REPORTING PERSONS  I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)  Flagship Pioneering Special Opportunities Fund II, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)  (a) ☐  (b) ☒
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION  Delaware

NUMBER OF SHARES  BENEFICIALLY  OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER  0
	6	SHARED VOTING POWER  0
	7	SOLE DISPOSITIVE POWER  0
	8	SHARED DISPOSITIVE POWER  0

9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON  0
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)  ☐
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9  0.0%
12	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)  PN

CUSIP No. 82657L 10 7 Page 5 of 11 Pages

1	NAMES OF REPORTING PERSONS  I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)  Flagship Pioneering Special Opportunities Fund II General Partner LLC
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)  (a) ☐  (b) ☒
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION  Delaware

NUMBER OF SHARES  BENEFICIALLY  OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER  0
	6	SHARED VOTING POWER  0
	7	SOLE DISPOSITIVE POWER  0
	8	SHARED DISPOSITIVE POWER  0

9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON  0
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)  ☐
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9  0.0%
12	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)  OO

CUSIP No. 82657L 10 7 Page 6 of 11 Pages

1	NAMES OF REPORTING PERSONS  I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)  Flagship Pioneering, Inc.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)  (a) ☐  (b) ☒
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION  Delaware

NUMBER OF SHARES  BENEFICIALLY  OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER  0
	6	SHARED VOTING POWER  0
	7	SOLE DISPOSITIVE POWER  0
	8	SHARED DISPOSITIVE POWER  0

9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON  0
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)  ☐
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9  0.0%
12	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)  CO

CUSIP No. 82657L 10 7 Page 7 of 11 Pages

1	NAMES OF REPORTING PERSONS  I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)  Noubar B. Afeyan, Ph.D.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)  (a) ☐  (b) ☒
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION  United States of America

NUMBER OF SHARES  BENEFICIALLY  OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER  0
	6	SHARED VOTING POWER  0
	7	SOLE DISPOSITIVE POWER  0
	8	SHARED DISPOSITIVE POWER  0

9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON  0
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)  ☐
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9  0.0%
12	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)  IN

CUSIP No. 82657L 10 7 Page 8 of 11 Pages

Item 1(a). Name of Issuer:

Sigilon Therapeutics, Inc. (the “Issuer”)

Item 1(b). Address of Issuer’s Principal Executive Offices:

100 Binney Street

Cambridge, MA 02142

Item 2(a). Names of Persons Filing:

The names of the persons filing this report (collectively, the “Reporting Persons”) are:

Flagship Ventures Fund V, L.P. (“Flagship Fund V”)

Flagship Ventures Fund V General Partner LLC (“Flagship V GP”)

Flagship Pioneering Special Opportunities Fund II, L.P. (“Flagship Opportunities Fund II”)

Flagship Pioneering Special Opportunities Fund II General Partner LLC (“Opportunities Fund II GP”)

Flagship Pioneering, Inc. (“Flagship Pioneering”)

Noubar B. Afeyan, Ph.D. (“Dr. Afeyan”)

Item 2(b). Address of Principal Business Office or, if None, Residence:

The address of the principal business office of each of the Reporting Persons is:

c/o Flagship Pioneering Inc.

55 Cambridge Parkway, Suite 800E

Cambridge, Massachusetts 02142

Item 2(c). Citizenship:

Flagship Fund V	Delaware
Flagship V GP	Delaware
Flagship Opportunities Fund II	Delaware
Opportunities Fund II GP	Delaware
Flagship Pioneering	Delaware
Dr. Afeyan	United States of America

Item 2(d). Title of Class of Securities:

Common Stock, $0.001 par value per share (“Common Stock”).

Item 2(e). CUSIP Number:

82657L 10 7

Item 3. If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

Not applicable.

CUSIP No. 82657L 10 7 Page 9 of 11 Pages

Item 4. Ownership.

As of August 11, 2023, each of the Reporting Persons no longer may be deemed to beneficially own any shares of Common Stock of the Issuer.

Item 5. Ownership of Five Percent or Less of a Class.

If this statement is being filed to report the fact that as of the date hereof the Reporting Persons have ceased to be the beneficial owner of more than five percent of the class of securities, check the following: ☒

Item 6. Ownership of More than Five Percent on Behalf of Another Person.

Not applicable.

Item 7. Identification and Classification of the SubsidiaryWhich Acquired the Security Being Reported on by the Parent Holding Company or Control Person.

Not applicable.

Item 8. Identification and Classification of Members of the Group.

Not applicable.

Item 9. Notice of Dissolution of Group.

Not applicable.

Item 10. Certification.

Not applicable.

CUSIP No. 82657L 10 7 Page 10 of 11 Pages

SIGNATURE

After reasonable inquiry and to the best of its knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

Date: February 14, 2024

FLAGSHIP VENTURES FUND V, L.P.
By:	Flagship Ventures Fund V General Partner LLC
	General Partner
	By:	/s/ Noubar B. Afeyan, Ph.D.
		Noubar B. Afeyan, Ph.D.
		Manager
FLAGSHIP VENTURES FUND V GENERAL PARTNER LLC
	By:	/s/ Noubar B. Afeyan, Ph.D.
		Noubar B. Afeyan, Ph.D.
		Manager
FLAGSHIP PIONEERING SPECIAL OPPORTUNITIES FUND II, L.P.
By:	Flagship Pioneering Special Opportunities Fund II General Partner LLC General Partner
	By: Flagship Pioneering, Inc., Manager
	By:	/s/ Noubar B. Afeyan, Ph.D.
		Noubar B. Afeyan, Ph.D.
		Chief Executive Officer
FLAGSHIP PIONEERING SPECIAL OPPORTUNITIES FUND II GENERAL PARTNER LLC
By:	Flagship Pioneering, Inc.
	Manager
	By:	/s/ Noubar B. Afeyan, Ph.D.
		Noubar B. Afeyan, Ph.D.
		Chief Executive Officer

CUSIP No. 82657L 10 7 Page 11 of 11 Pages

FLAGSHIP PIONEERING, INC.
	By:	/s/ Noubar B. Afeyan, Ph.D.
		Noubar B. Afeyan, Ph.D.
		Chief Executive Officer
/s/ Noubar B. Afeyan, Ph.D.
Noubar B. Afeyan, Ph.D.