[SCHEDULE 13D/A] Jade Biosciences, Inc. SEC Filing

Rhea-AI Filing Summary

Fairmount Funds Management LLC and related entities reported beneficial ownership of 10,353,875 shares of Jade Biosciences, Inc. common stock, representing 19.99% of the outstanding class as calculated on an adjusted share count of 51,795,275 shares. The stake includes 4,553,494 currently outstanding shares, Pre-Funded Warrants exercisable for 46,381 shares subject to a 9.99% ownership cap, and conversion rights to 5,754,000 shares from Series A non-voting preferred stock subject to a 19.99% cap. On 10/06/2025, Fund II purchased 1,333,126 shares at $9.14 per share and Pre-Funded Warrants for 855,047 shares at $9.1399 each, for an aggregate purchase price of $19,999,815.72, paid from working capital. The PIPE closed on 10/08/2025, and the investors received registration rights requiring the company to file a resale registration statement within 45 days of closing (or earlier) with a target effectiveness within 75 days after filing, subject to specified exceptions and penalties.

Positive

• Acquisition of a substantial economic position: Fairmount now controls 10,353,875 shares representing 19.99% of the class.
• PIPE provides immediate capital: Fund II invested $19,999,815.72 in the private placement, strengthening investor commitment.
• Registration rights granted: Investors secured registration within 45 days and targeted effectiveness within 75 days, enabling resale liquidity when effective.

Negative

• Beneficial ownership caps limit immediate exercise: Pre-Funded Warrant exercises are limited to 9.99% and preferred conversions to 19.99%, restricting full share realization.
• Near-20% stake may prompt market or governance scrutiny: Shared voting power at 19.99% could attract attention from other shareholders and the company.
• Registration timetable creates execution risk: The company must file within 45 days and seek effectiveness within 75 days, with penalties if delayed.

Insights

Private Equity / PIPE Structuring Analyst

Fairmount acquired a near-20% economic stake via a $20.0M PIPE with capped exercisability.

The transaction combines direct common shares, Pre-Funded Warrants and convertible preferred stock to deliver immediate ownership while embedding 9.99% and 19.99% beneficial ownership caps that limit future dilution and exercise timing. The use of pre-funded warrants is a common mechanism to allow investors to acquire economic exposure while avoiding immediate reporting or ownership thresholds.

Key dependencies include the registration rights timetable—Company must file within 45 days and use reasonable best efforts to have effectiveness within 75 days—and the beneficial ownership limits, which can be adjusted to 19.99% only after 61 days notice. Watch the registration filing and any changes in outstanding share count that affect the stated 19.99% threshold over the next 1-3 months.

Investor Relations / Governance Specialist

Disclosure clarifies voting/dispositive control and registration obligations tied to the PIPE.

The filing shows Fairmount has shared voting and dispositive power over 10,353,875 shares, which should be reflected in governance communications and may trigger stakeholder engagement given near-20% holding. The registration rights agreement shifts resale liquidity timing risk from investors to the company and includes expense indemnities for holders.

Monitor whether the company files the registration statement within the 45-day deadline and whether any subsequent ownership changes trigger automatic adjustments to the beneficial ownership limitation. Any delay or failure could impose contractual penalties or affect secondary market liquidity for these shares within the next 2-3 months.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 1)

Jade Biosciences, Inc.

(Name of Issuer)

Common Stock, $0.0001 par value

(Title of Class of Securities)

008064206

(CUSIP Number)

Ms. Erin O'Connor
Fairmount Funds Management LLC, 200 Barr Harbor Drive, Suite 400
West Conshohocken, PA, 19428
(267) 262-5300

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

10/06/2025

(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. 008064206

1

Name of reporting person

Fairmount Funds Management 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 10,353,875.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 10,353,875.00

11

Aggregate amount beneficially owned by each reporting person

10,353,875.00

12

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

13

Percent of class represented by amount in Row (11)

19.99 %

14

Type of Reporting Person (See Instructions)

IA

Comment for Type of Reporting Person:

The securities (a) include (i) 1,897,677 shares of common stock, $0.0001 par value per share (the "Common Stock"), 46,381 shares of Common Stock issuable upon the exercise of Pre-Funded Warrants and 5,754,000 shares of Common Stock issuable upon conversion of 5,754 shares of Series A non-voting convertible preferred stock, par value $0.0001 per share (the "Series A Preferred Stock"), directly held by Fairmount Healthcare Fund II L.P., a Delaware limited partnership ("Fund II"), and (ii) 2,655,817 shares of Common Stock directly held by Fairmount Healthcare Co-Invest IV L.P., a Delaware limited partnership ("Co-Invest"), and (b) exclude (i) 5,743,825 shares of Common Stock issuable upon exercise of Pre-Funded Warrants and (ii) 6,868,000 shares of Common Stock issuable upon conversion of 6,868 shares of Series A Preferred Stock, in each case directly held by Fund II, as the exercise of the Pre-Funded Warrants is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock and the conversion of the Series A Preferred Stock is subject to a beneficial ownership limitation of 19.99%. At such time as Fairmount Funds Management LLC, a Delaware limited liability company and Securities and Exchange Commission registered investment adviser under the Investment Advisers Act of 1940 ("Fairmount"), and its affiliates beneficially own 9.0% or less of the Common Stock, the beneficial ownership limitation with respect to the Series A Preferred Stock will automatically reduce to 9.99%. Row 13 is based on 51,795,275 shares of Common Stock outstanding as of October 8, 2025, consisting of (i) 32,626,730 shares of Common Stock outstanding as of August 8, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) a total of 13,368,164 shares of Common Stock issued in the Company's private placement on October 6, 2025, as reported in the Company's Current Report on Form 8-K filed on October 7, 2025, (iii) the 46,381 shares of Common Stock underlying the Pre-Funded Warrants, and (iv) the 5,754,000 shares of Common Stock underlying the 5,754 shares of Series A Preferred Stock owned by the Reporting Persons, subject to the respective beneficial ownership limitations.

SCHEDULE 13D

CUSIP No. 008064206

1

Name of reporting person

Fairmount Healthcare 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 7,698,058.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 7,698,058.00

11

Aggregate amount beneficially owned by each reporting person

7,698,058.00

12

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

13

Percent of class represented by amount in Row (11)

14.86 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The securities (a) include 1,897,677 shares of Common Stock, 46,381 shares of Common Stock issuable upon the exercise of Pre-Funded Warrants, and 5,754,000 shares of Common Stock issuable upon conversion of 5,754 shares of Series A Preferred Stock held directly by Fund II and (b) exclude 5,743,825 shares of Common Stock issuable upon exercise of Pre-Funded Warrants and 6,868,000 shares of Common Stock issuable upon conversion of 6,868 shares of Series A Preferred Stock, as the exercise of the Pre-Funded Warrants is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock and the conversion of the Series A Preferred Stock is subject to a beneficial ownership limitation of 19.99%. At such time as Fairmount and its affiliates beneficially own 9.0% or less of the Common Stock, the beneficial ownership limitation with respect to the Series A Preferred Stock will automatically reduce to 9.99%. Row 13 is based on 51,795,275 shares of Common Stock outstanding as of October 8, 2025, consisting of (i) 32,626,730 shares of Common Stock outstanding as of August 8, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) a total of 13,368,164 shares of Common Stock issued in the Company's private placement on October 6, 2025, as reported in the Company's Current Report on Form 8-K filed on October 7, 2025, (iii) the 46,381 shares of Common Stock underlying the Pre-Funded Warrants, and (iv) the 5,754,000 shares of Common Stock underlying the 5,754 shares of Series A Preferred Stock owned by Fund II, subject to the respective beneficial ownership limitations.

SCHEDULE 13D

CUSIP No. 008064206

1

Name of reporting person

Fairmount Healthcare Co-Invest IV 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 2,655,817.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 2,655,817.00

11

Aggregate amount beneficially owned by each reporting person

2,655,817.00

12

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

13

Percent of class represented by amount in Row (11)

5.77 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The securities include 2,655,817 shares of Common Stock held directly by Co-Invest. Row 13 is based on 45,994,894 shares of Common Stock outstanding as of October 8, 2025, consisting of (i) 32,626,730 shares of Common Stock outstanding as of August 8, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, and (ii) a total of 13,368,164 shares of Common Stock issued in the Company's private placement on October 6, 2025, as reported in the Company's Current Report on Form 8-K filed on October 7, 2025.

SCHEDULE 13D

CUSIP No. 008064206

1

Name of reporting person

Peter Evan Harwin

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

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 10,353,875.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 10,353,875.00

11

Aggregate amount beneficially owned by each reporting person

10,353,875.00

12

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

13

Percent of class represented by amount in Row (11)

19.99 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

The information in the "Comments" to the cover page for Fairmount Funds Management LLC above is hereby incorporated by reference.

SCHEDULE 13D

CUSIP No. 008064206

1

Name of reporting person

Tomas Kiselak

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

SLOVAKIA

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 10,353,875.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 10,353,875.00

11

Aggregate amount beneficially owned by each reporting person

10,353,875.00

12

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

13

Percent of class represented by amount in Row (11)

19.99 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

The information in the "Comments" to the cover page for Fairmount Funds Management LLC above is hereby incorporated by reference.

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock, $0.0001 par value
(b)	Name of Issuer: Jade Biosciences, Inc.
(c)	Address of Issuer's Principal Executive Offices: 221 Crescent Street, Building 23, Suite 105, Waltham, MASSACHUSETTS , 02453.
Item 1 Comment: This Amendment No. 1 amends and supplements the statement on Schedule 13D originally filed with the Securities and Exchange Commission (the "SEC") on May 1, 2025 (as amended, the "Statement" or the "Schedule 13D") by the Reporting Persons with respect to the Common Stock of the Company. Unless otherwise defined herein, capitalized terms used in this Amendment No. 1 shall have the meanings ascribed to them in the Statement. Unless amended or supplemented below, the information in the Statement remains unchanged.
Item 3.	Source and Amount of Funds or Other Consideration
	Item 3 of the Statement is hereby amended and supplemented as follows: In aggregate, the Reporting Persons have voting and dispositive power over 10,353,875 shares of Common Stock of the Company, which is comprised of (a) 4,553,494 shares of Common Stock, (b) Pre-Funded Warrants to purchase up to 46,381 shares of Common Stock, the exercise of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock (and excluding shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation), and (c) 5,754,000 shares of Common Stock issuable upon conversion of 5,754 shares of Series A Preferred Stock, the conversion of which is subject to a beneficial ownership limitation of 19.99% of the outstanding Common Stock (and excluding shares of Common Stock issuable upon conversion of the Series A Preferred Stock in excess of the beneficial ownership limitation). On October 6, 2025, Fund II purchased a total of (i) 1,333,126 shares of Common Stock and (ii) Pre-Funded Warrants to purchase up to 855,047 shares of Common Stock through a private placement with the Company for an aggregate purchase price of $19,999,815.72. The securities were purchased with working capital.
Item 4.	Purpose of Transaction
	Item 4 of the Statement is hereby amended and supplemented as follows: Subscription Agreement On October 6, 2025, the Company entered into a securities purchase agreement (the "Subscription Agreement") for a private investment in public equity ("PIPE") with certain institutional and accredited investors, including Fund II. The PIPE closed on October 8, 2025. Pursuant to the terms of the Subscription Agreement, Fund II purchased a total of 1,333,126 shares of Common Stock at a purchase price of $9.14 per share and Pre-Funded Warrants to purchase 855,047 shares of Common Stock at a purchase price of $9.1399 per Pre-Funded Warrant, which represents the per share purchase price of the shares of Common Stock less the $0.0001 per share exercise price for each Pre-Funded Warrant. The foregoing description of the Subscription Agreement does not purport to be complete and is qualified in its entirety by reference to the form of the Subscription Agreement, which is filed as Exhibit 99.2 to this Schedule 13D and incorporated herein by reference. Pre-Funded Warrants The Pre-Funded Warrants purchased by Fund II in the PIPE will be exercisable at any time after the date of issuance. A holder of Pre-Funded Warrants may not exercise the warrant if the holder, together with its affiliates, would beneficially own more than 9.99%, as applicable, of the number of shares of Common Stock outstanding immediately after giving effect to such exercise. A holder of Pre-Funded Warrants may increase or decrease this percentage to a percentage not in excess of 19.99% by providing at least 61 days' prior notice to the Company. The foregoing description of the Pre-Funded Warrants does not purport to be complete and is qualified in its entirety by reference to the form of Pre-Funded Warrant, which is filed as Exhibit 99.3 to this Schedule 13D and incorporated herein by reference Registration Rights Agreement In connection with the PIPE, the Company and investors participating in the PIPE, including Fund II, entered into a registration rights agreement (the "Registration Rights Agreement"), which provides that the Company will register the resale of the shares of Common Stock and the shares of Common Stock underlying the Pre-Funded Warrants sold in the PIPE. Common Shares and Warrant Shares. The Company is required to prepare and file a registration statement with the Securities and Exchange Commission no later than the earlier of (a) 45 days following the closing of the PIPE and (b) the business day following the date on which the Company files its Quarterly Report on Form 10-Q for the quarter ended September 30, 2025, and to use its reasonable best efforts to have the registration statement declared effective 75 days thereafter, subject to certain exceptions and specified penalties if timely effectiveness is not achieved. The Registration Rights Agreement also provides that the Company is required to pay certain expenses relating to such registrations and indemnify the applicable securityholders against certain liabilities. The foregoing summary of the Registration Rights Agreement does not purport to be complete and is qualified in its entirety by reference to the form of Registration Rights Agreement, which is filed as Exhibit 99.4 to this Schedule 13D.
Item 5.	Interest in Securities of the Issuer
(a)	The information in rows 11 and 13 of each of the cover pages (including the explanatory note in the "Comments") of this Amendment No. 1 is incorporated by reference herein.
(b)	The information in rows 7 through 10 of each of the cover pages of this Amendment No. 1 is incorporated by reference herein. The Reporting Persons' securities (a) include (i) 1,897,677 shares of Common Stock, 46,381 shares of Common Stock issuable upon the exercise of Pre-Funded Warrants and 5,754,000 shares of Common Stock issuable upon conversion of 5,754 shares of Series A Preferred Stock, directly held by Fund II, and (ii) 2,655,817 shares of Common Stock directly held by Co-Invest, and (b) exclude (i) 5,743,825 shares of Common Stock issuable upon exercise of Pre-Funded Warrants and (ii) 6,868,000 shares of Common Stock issuable upon conversion of 6,868 shares of Series A Preferred Stock, in each case directly held by Fund II. The exercise of the Pre-Funded Warrants is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock and the conversion of the Series A Preferred Stock is subject to a beneficial ownership limitation of 19.99%. The securities exclude shares of Common Stock issuable upon exercise of Pre-Funded Warrants and conversion of shares of Series A Preferred Stock in excess of such beneficial ownership limitations. At such time as Fairmount and its affiliates beneficially own 9.0% or less of the Common Stock, the beneficial ownership limitation with respect to the Series A Preferred Stock will automatically reduce to 9.99%.
(c)	On October 6, 2025, Fund II purchased a total of (i) 1,333,126 shares of Common Stock at a price of $9.14 per share and (ii) Pre-Funded Warrants to purchase 855,047 shares of Common Stock at a purchase price of $9.1399 per Pre-Funded Warrant, which represents the per share purchase price of the shares of Common Stock less the $0.0001 per share exercise price for each Pre-Funded Warrant, in the PIPE for an aggregate purchase price of $19,999,815.72.
(d)	Fairmount is the investment manager to Fund II and Co-Invest and has voting and dispositive power over shares of Common Stock held on behalf of Fund II and Co-Invest.
Item 6.	Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer
	The information set forth in Item 4 of this Amendment No. 1 is incorporated herein by reference.
Item 7.	Material to be Filed as Exhibits.
	99.1 Joint Filing Agreement 99.2 Subscription Agreement (incorporated by reference to Exhibit 10.1 of the Company's Form 8-K filed with the SEC on October 7, 2025). 99.3 Form of Pre-Funded Warrant (incorporated by reference to Exhibit 4.1 of the Company's Form 8-K filed with the SEC on October 7, 2025). 99.4 Form of Registration Rights Agreement (incorporated by reference to Exhibit 10.2 of the Company's Form 8-K filed with the SEC on October 7, 2025).

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.

Fairmount Funds Management LLC

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin, Managing Member
Date:	10/08/2025

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	10/08/2025

Fairmount Healthcare Fund II L.P.

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin, Managing Member
Date:	10/08/2025

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	10/08/2025

Fairmount Healthcare Co-Invest IV L.P.

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin, Managing Member
Date:	10/08/2025

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	10/08/2025

Peter Evan Harwin

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin
Date:	10/08/2025

Tomas Kiselak

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak
Date:	10/08/2025

FAQ

What stake does Fairmount hold in Jade Biosciences (JBIO)?

Fairmount and related entities beneficially own 10,353,875 shares, equal to 19.99% of outstanding common stock on the adjusted share count used in the filing.

How much did Fairmount pay in the PIPE transaction?

Fund II paid an aggregate of $19,999,815.72 on 10/06/2025 for 1,333,126 shares at $9.14 and Pre-Funded Warrants for 855,047 shares at $9.1399 each.

Are there limits on exercising the Pre-Funded Warrants or converting preferred stock?

Yes. Pre-Funded Warrants cannot be exercised if doing so would result in beneficial ownership above 9.99%, and Series A preferred conversions are limited to 19.99%; these caps can be adjusted per the agreement with notice.

When must the company register the PIPE shares for resale?

The registration statement must be filed no later than the earlier of 45 days after PIPE closing or the business day after the company files its quarterly report for the period ending 9/30/2025, and the company will use reasonable best efforts to have it declared effective within 75 days after filing.

Who holds voting and dispositive power over these shares?

Fairmount Funds Management LLC, as investment manager, has shared voting and dispositive power over the shares held by Fund II and Co-Invest, per the filing.