[SCHEDULE 13D/A] Disc Medicine, Inc. SEC Filing

Rhea-AI Filing Summary

Atlas Venture and affiliated funds reported holdings in Disc Medicine, Inc. This Amendment No. 4 to a prior Schedule 13D discloses that multiple Atlas Venture funds collectively beneficially own 2,148,984 shares of Disc Medicine common stock, representing an estimated 6.2% of outstanding shares based on 34,765,581 shares outstanding as of July 31, 2025. Individual fund holdings are reported as Atlas Venture Fund X: 1,084,527 shares (3.1%), Atlas Venture Fund XII: 51,000 shares (0.1%), Atlas Venture Opportunity Fund I: 592,908 shares (1.7%), and Atlas Venture Opportunity Fund II: 420,549 shares (1.2%). The filing states these reporting persons have shared voting and dispositive power over their respective shares and notes they "may be deemed" a group but explicitly disclaim group membership. The amendment references recent transactions in Exhibit 99.1 and updates prior Schedule 13D disclosures.

Positive

• Clear quantitative disclosure of aggregate and per-fund holdings (2,148,984 shares; 6.2% aggregate).
• Breakdown by fund provided with specific share counts and percentage of class for each reporting person.
• Disclosure of voting/dispositive power (shared control specified) and reference to recent transactions in Exhibit 99.1.

Negative

• None.

Insights

Financial Analyst

TL;DR: Atlas Venture-affiliated funds disclosed a combined 6.2% stake (2,148,984 shares) in DISC Medicine, a material passive stake for investors to note.

The filing provides a clear, quantified ownership picture across multiple Atlas-managed vehicles, showing concentrated positions split among four fund groupings. The aggregate 6.2% stake is sizeable relative to the issuer's 34.8 million shares outstanding and could influence liquidity and block-trade dynamics. The reporting of shared voting and dispositive power indicates centralized control within the Atlas structure, while the explicit disclaimer of group membership limits legal inferences about coordinated action. The filing’s reference to Exhibit 99.1 for recent transactions is useful for traceability of how the position changed in the prior 60 days.

Corporate Governance Expert

TL;DR: The Schedule 13D/A discloses shared voting/dispositive power over a combined 6.2% position and a disclaimer of group status.

The amendment updates beneficial ownership across affiliated funds and reiterates that general partner entities exercise shared control. From a governance perspective, disclosure of shared voting/dispositive power is important for board engagement and proxy considerations. The filing’s explicit group-disclaimer is a noteworthy legal posture: it preserves flexibility while providing stakeholders transparency on who exercises influence. The inclusion of a joint filing agreement as an exhibit supports coordinated reporting compliance.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 4)

DISC MEDICINE, INC.

(Name of Issuer)

Common Stock, $0.0001 par value per share

(Title of Class of Securities)

254604101

(CUSIP Number)

Attention: Ommer Chohan, CFO
Atlas Venture, 300 Technology Square, 8th Floor
Cambridge, MA, 02139
(857) 201-2700

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

08/13/2025

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.

The information required on 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).

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

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

Source of funds (See Instructions)

WC

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 1,084,527.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 1,084,527.00

11

Aggregate amount beneficially owned by each reporting person

1,084,527.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

3.1 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

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

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 1,084,527.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 1,084,527.00

11

Aggregate amount beneficially owned by each reporting person

1,084,527.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

3.1 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

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

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 1,084,527.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 1,084,527.00

11

Aggregate amount beneficially owned by each reporting person

1,084,527.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

3.1 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

Atlas Venture Fund XII, L.P.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

WC

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 51,000.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 51,000.00

11

Aggregate amount beneficially owned by each reporting person

51,000.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

0.1 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

Atlas Venture Associates XII, L.P.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 51,000.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 51,000.00

11

Aggregate amount beneficially owned by each reporting person

51,000.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

0.1 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

Atlas Venture Associates XII, LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 51,000.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 51,000.00

11

Aggregate amount beneficially owned by each reporting person

51,000.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

0.1 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

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

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 592,908.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 592,908.00

11

Aggregate amount beneficially owned by each reporting person

592,908.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

1.7 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

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

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 592,908.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 592,908.00

11

Aggregate amount beneficially owned by each reporting person

592,908.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

1.7 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

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

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 592,908.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 592,908.00

11

Aggregate amount beneficially owned by each reporting person

592,908.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

1.7 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

Atlas Venture Opportunity Fund II, L.P.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 420,549.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 420,549.00

11

Aggregate amount beneficially owned by each reporting person

420,549.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

1.2 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

Atlas Venture Associates Opportunity II, LP

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 420,549.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 420,549.00

11

Aggregate amount beneficially owned by each reporting person

420,549.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

1.2 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 254604101

1

Name of reporting person

Atlas Venture Associates Opportunity II, LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 420,549.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 420,549.00

11

Aggregate amount beneficially owned by each reporting person

420,549.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

1.2 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock, $0.0001 par value per share
(b)	Name of Issuer: DISC MEDICINE, INC.
(c)	Address of Issuer's Principal Executive Offices: 321 ARSENAL STREET, SUITE 101, WATERTOWN, MASSACHUSETTS , 02472.
Item 1 Comment: This Amendment No. 4 to Schedule 13D ("Amendment No. 4") is being filed as an amendment to the statement on Schedule 13D relating to common stock, par value $0.0001 per share of Disc Medicine, Inc., a Delaware corporation, as filed with the Securities and Exchange Commission (the "SEC") on February 16, 2021 (the "Initial Schedule 13D") and later amended on February 14, 2023 ("Amendment No. 1") , September 21, 2023 ("Amendment No. 2") and January 24, 2024 ("Amendment No. 3" and together with the Initial Schedule 13D, Amendment No. 1 and Amendment No. 2 the "Prior Schedule 13D"). All capitalized terms not otherwise defined herein shall have the meanings ascribed to the terms in the Prior Schedule 13D. The Prior Schedule 13D is hereby amended and supplemented as follows and, except as expressly amended below, the Prior Schedule 13D remains in full force and effect.
Item 5.	Interest in Securities of the Issuer
(a)	As of the date hereof, Atlas X is the record owner of 1,084,527 shares of Common Stock. AVA X LP is the general partner of Atlas X and AVA X LLC is the general partner of AVA X LP. Each of Atlas X, AVA X LP and AVA X LLC has shared voting and dispositive power over the shares held by Atlas X. As such, each of Atlas X, AVA X LP and AVA X LLC may be deemed to beneficially own the shares held by Atlas X. As of the date hereof, Atlas XII is the record owner of 51,000 shares of Common Stock. AVA XII LP is the general partner of Atlas XII and AVA XII LLC is the general partner of AVA XII LP. Each of Atlas XII, AVA XII LP and AVA XII LLC has shared voting and dispositive power over the shares held by Atlas XII. As such, each of Atlas XII, AVA XII LP and AVA XII LLC may be deemed to beneficially own the shares held by Atlas XII. As of the date hereof, AVOF is the record owner of 592,908 shares of Common Stock. AVAO LP is the general partner of AVOF and AVAO LLC is the general partner of AVAO LP. Each of AVOF, AVAO LP and AVAO LLC has shared voting and dispositive power over the shares held by AVOF. As such, each of AVOF, AVAO LP and AVAO LLC may be deemed to beneficially own the shares held by AVOF. As of the date hereof, AVOF II is the record owner of 420,549 shares of Common Stock. AVAO II LP is the general partner of AVOF II and AVAO II LLC is the general partner of AVAO II LP. Each of AVOF II, AVAO II LP and AVAO II LLC has shared voting and dispositive power over the shares held by AVOF II. As such, each of AVOF II, AVAO II LP and AVAO II LLC may be deemed to beneficially own the shares held by AVOF II. Each of the Fund X Reporting Persons, Fund XII Reporting Persons, Opportunity Fund Reporting Persons and Opportunity Fund II Reporting Persons may be deemed to beneficially own 3.1%, 0.1%, 1.7% and 1.2%, respectively, of the Issuer's outstanding Common Stock, which percentages are calculated based upon 34,765,581 shares of Common Stock outstanding as of July 31, 2025, as reported in the Issuer's Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 for the quarterly period ended June 30 on Form 10-Q, filed with the Securities and Exchange Commission on August 7, 2025. Collectively, the Reporting Persons beneficially own an aggregate of 2,148,984 shares of Common Stock, which represents an estimated 6.2% of the Issuer's outstanding Common Stock. The Fund X Reporting Persons, Fund XII Reporting Persons, Opportunity Fund Reporting Persons and the Opportunity Fund II Reporting Persons are under common control and as a result, the Reporting Persons may be deemed to be members of a group. However, the Reporting Persons disclaim such group membership, and this Schedule 13D shall not be deemed an admission that the Reporting Persons are members of a group for purposes of Section 13 or for any other purposes.
(b)	As of the date hereof, each of Atlas X, AVA X LP and AVA X LLC have shared voting power and shared dispositive power over 1,084,527 shares of Common Stock. As of the date hereof, each of Atlas XII, AVA XII LP and AVA XII LLC have shared voting power and shared dispositive power over 51,000 shares of Common Stock. As of the date hereof, each of AVOF, AVAO LP and AVAO LLC have shared voting power and shared dispositive power over 592,908 shares of Common Stock. As of the date hereof, each of AVOF II, AVAO II LP and AVAO II LLC have shared voting power and shared dispositive power over 420,549 shares of Common Stock.
(c)	The transactions in the common stock by the Reporting Persons during the past sixty days are set forth in Exhibit 99.1 and are incorporated herein by reference.
Item 7.	Material to be Filed as Exhibits.
	Joint Filing Agreement

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 general partner, By: Atlas Venture Associates X, LLC, its general partner, By Ommer Chohan, CFO
Date:	08/15/2025

ATLAS VENTURE ASSOCIATES X, L.P.

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates X, LLC, its general partner, By Ommer Chohan, CFO
Date:	08/15/2025

Atlas Venture Associates X, LLC

Signature:	/s/ Ommer Chohan
Name/Title:	Ommer Chohan, CFO
Date:	08/15/2025

Atlas Venture Fund XII, L.P.

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates XII, L.P., its general partner, By: Atlas Venture Associates XII, LLC, its general partner, By Ommer Chohan, CFO
Date:	08/15/2025

Atlas Venture Associates XII, L.P.

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates XII, LLC, its general partner, By Ommer Chohan, CFO
Date:	08/15/2025

Atlas Venture Associates XII, LLC

Signature:	/s/ Ommer Chohan
Name/Title:	Ommer Chohan, CFO
Date:	08/15/2025

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:	08/15/2025

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:	08/15/2025

Atlas Venture Associates Opportunity I, LLC

Signature:	/s/ Ommer Chohan
Name/Title:	Ommer Chohan, CFO
Date:	08/15/2025

Atlas Venture Opportunity Fund II, L.P.

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates Opportunity II, L.P., its GP, By: Atlas Venture Associates Opportunity II, LLC, its GP, By Ommer Chohan, CFO
Date:	08/15/2025

Atlas Venture Associates Opportunity II, LP

Signature:	/s/ Ommer Chohan
Name/Title:	By: Atlas Venture Associates Opportunity II, LLC, its GP, By Ommer Chohan, CFO
Date:	08/15/2025

Atlas Venture Associates Opportunity II, LLC

Signature:	/s/ Ommer Chohan
Name/Title:	Ommer Chohan, CFO
Date:	08/15/2025

FAQ

Who filed the Schedule 13D/A for Disc Medicine (IRON)?

The filing was made by multiple Atlas Venture entities, including Atlas Venture Fund X, L.P.; Atlas Venture Fund XII, L.P.; Atlas Venture Opportunity Fund I, L.P.; and Atlas Venture Opportunity Fund II, L.P.

How many Disc Medicine shares do the reporting persons beneficially own?

Collectively the reporting persons beneficially own 2,148,984 shares of Disc Medicine common stock.

What percentage of Disc Medicine does the reported holding represent?

The aggregate position represents an estimated 6.2% of outstanding common stock based on 34,765,581 shares outstanding as of July 31, 2025.

How are the holdings split among the Atlas funds?

Reported holdings are: Atlas Venture Fund X: 1,084,527 shares (3.1%), Fund XII: 51,000 shares (0.1%), Opportunity Fund I: 592,908 shares (1.7%), Opportunity Fund II: 420,549 shares (1.2%).

Do the reporting persons have voting control over the shares?

Yes; the filing states the named reporting persons have shared voting and shared dispositive power over the shares held by each fund.