Atlas Venture amends 13G/A on GBIO holdings (GBIO)

Rhea-AI Filing Summary

Atlas Venture reporting persons filed Amendment No. 3 to a Schedule 13G/A stating they beneficially own 0 shares (0%) of Generation Bio Co. common stock (CUSIP 37148K100). The filing lists the Reporting Persons and principal address and is signed on 05/14/2026.

Key Figures

Amendment: Amendment No. 3 CUSIP: 37148K100 Shares beneficially owned: 0 shares +2 more

5 metrics

Amendment Amendment No. 3 Schedule 13G/A filing

CUSIP 37148K100 Generation Bio Co. Common Stock

Shares beneficially owned 0 shares Amount beneficially owned reported in Item 4(a)

Percent of class 0% Percent of class reported in Item 4(b)

Signature date 05/14/2026 Date signatures were executed

Key Terms

Schedule 13G/A, beneficially owned, Joint Filing Agreement

3 terms

Schedule 13G/A regulatory

"filed Amendment No. 3 to a Schedule 13G/A stating beneficial ownership"

A Schedule 13G/A is an amended public filing with the U.S. securities regulator that updates a previous Schedule 13G, disclosing when an individual or group holds a substantial (typically over 5%) stake in a company and is claiming a passive, non‑controlling intent. Investors monitor these updates because rising or falling holdings can signal changing confidence, potential future moves, or shifts in voting power — like watching a public ledger where large shareholders quietly adjust their positions.

beneficially owned financial

"Amount beneficially owned: 0 (b) Percent of class: 0 %"

Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.

Joint Filing Agreement regulatory

"Exhibit Information JOINT FILING AGREEMENT"

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Accepted by SEC EDGAR 05/14/2026 - 04:24 PM Learn about SEC filing dates

FAQ

What does Atlas Venture's Schedule 13G/A amendment for GBIO state?

It reports zero beneficial ownership of GBIO common stock. The amendment lists multiple Atlas entities as Reporting Persons and confirms 0 shares (0%) beneficially owned, with signatures dated 05/14/2026.

Which Atlas entities filed the amendment reporting ownership in GBIO?

Multiple Atlas Venture entities filed as Reporting Persons. They include Atlas Venture Fund X, L.P.; Atlas Venture Associates X, L.P.; Atlas Venture Associates X, LLC; Atlas Venture Opportunity Fund I, L.P.; Atlas Venture Associates Opportunity I, L.P.; and Atlas Venture Associates Opportunity I, LLC.

What CUSIP and class are covered in the filing for Generation Bio (GBIO)?

The filing covers GBIO Common Stock, CUSIP 37148K100. The title of the class is listed as Common Stock, $0.0001 par value, with the issuer identified as Generation Bio Co., Cambridge, MA.

When was the Schedule 13G/A amendment signed and who signed it?

The amendment is signed on 05/14/2026 by Ommer Chohan. Signatures show Ommer Chohan signing on behalf of multiple Atlas entities in the capacity indicated in the filing.

Does the filing indicate any voting or dispositive power held by the Reporting Persons?

No voting or dispositive power is reported. The filing lists 0 for sole and shared voting power and 0 for sole and shared dispositive power in each Reporting Person's ownership table.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 3)

GENERATION BIO CO.

(Name of Issuer)

COMMON STOCK, $0.0001 PAR VALUE

(Title of Class of Securities)

37148K100

(CUSIP Number)

03/31/2026

(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)

schemaVersion:

SCHEDULE 13G

CUSIP Number(s): 37148K100

1

Names of Reporting Persons

Atlas Venture Fund X, 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 0.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

0.00

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

SCHEDULE 13G

CUSIP Number(s): 37148K100

1

Names of Reporting Persons

ATLAS VENTURE ASSOCIATES X, 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 0.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

0.00

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

SCHEDULE 13G

CUSIP Number(s): 37148K100

1

Names of Reporting Persons

Atlas Venture Associates X, 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 0.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

0.00

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

SCHEDULE 13G

CUSIP Number(s): 37148K100

1

Names of Reporting Persons

Atlas Venture Opportunity Fund I, 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 0.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

0.00

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

SCHEDULE 13G

CUSIP Number(s): 37148K100

1

Names of Reporting Persons

Atlas Venture Associates Opportunity I, 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 0.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

0.00

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

SCHEDULE 13G

CUSIP Number(s): 37148K100

1

Names of Reporting Persons

Atlas Venture Associates Opportunity I, 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 0.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

0.00

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

SCHEDULE 13G

Item 1.
(a)	Name of issuer: GENERATION BIO CO.
(b)	Address of issuer's principal executive offices: 301 BINNEY STREET, CAMBRIDGE, MA, 02142.
Item 2.
(a)	Name of person filing: This Schedule 13G is filed by (i) Atlas Venture Fund X, L.P., a Delaware limited partnership ("Atlas X"), (ii) Atlas Venture Associates X, L.P., a Delaware limited partnership ("AVA X LP"), (iii) Atlas Venture Associates X, LLC, a Delaware limited liability company ("AVA X LLC" and together with Atlas X and AVA X LP, the "Fund X Reporting Persons"), (iv) Atlas Venture Opportunity Fund I, L.P., a Delaware limited partnership ("AVO I"), (v) Atlas Venture Associates Opportunity I, L.P., a Delaware limited partnership ("AVAO LP") and (vi) Atlas Venture Associates Opportunity I, LLC, a Delaware limited liability company ("AVAO LLC" and together with AVO I and AVAO LP, the "Opportunity Fund Reporting Persons" and together with the Fund X Reporting Persons, the "Reporting Persons").
(b)	Address or principal business office or, if none, residence: 300 Technology Square, 8th Floor Cambridge, Massachusetts 02139
(c)	Citizenship: Each of Atlas X, AVA X LP, AVO I and AVAO LP is a Delaware limited partnership. Each of AVA X LLC and AVAO LLC is a Delaware limited liability company.
(d)	Title of class of securities: COMMON STOCK, $0.0001 PAR VALUE
(e)	CUSIP No.: 37148K100
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:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: 0
(b)	Percent of class: 0 %
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 0
	(ii) Shared power to vote or to direct the vote: 0
	(iii) Sole power to dispose or to direct the disposition of: 0
	(iv) Shared power to dispose or to direct the disposition of: 0
Item 5.	Ownership of 5 Percent or Less of a Class.
	Ownership of 5 percent or less of a class
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	Not Applicable
Item 7.	Identification and Classification of the Subsidiary Which 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.	Certifications:
	Not Applicable

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Atlas Venture Fund X, L.P.

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates X, L.P., its GP, By: Atlas Venture Associates X, LLC, its GP, By: Ommer Chohan, CFO
Date:	05/14/2026

ATLAS VENTURE ASSOCIATES X, L.P.

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates X, LLC, its GP, By: Ommer Chohan, CFO
Date:	05/14/2026

Atlas Venture Associates X, LLC

Signature:	/s/ Ommer Chohan
Name/Title:	Ommer Chohan, CFO
Date:	05/14/2026

Atlas Venture Opportunity Fund I, L.P.

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates Opportunity I, L.P., its GP, By: Atlas Venture Associates Opportunity I, LLC, its GP, By: Ommer Chohan, CFO
Date:	05/14/2026

Atlas Venture Associates Opportunity I, L.P.

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates Opportunity I, LLC, its GP, By: Ommer Chohan, CFO
Date:	05/14/2026

Atlas Venture Associates Opportunity I, LLC

Signature:	/s/ Ommer Chohan
Name/Title:	Ommer Chohan, CFO
Date:	05/14/2026

Exhibit Information

JOINT FILING AGREEMENT