Fairmount funds lock up 19.99% stake in Jade Biosciences (JBIO)

Rhea-AI Filing Summary

Fairmount Funds and affiliated entities report beneficial ownership of 14,069,494 shares of Jade Biosciences common stock, representing 19.99% of the class. This stake includes 1,897,677 common shares and 9,516,000 shares issuable upon conversion of Series A non-voting convertible preferred stock held by Fairmount Healthcare Fund II L.P., plus 2,655,817 common shares held by Fairmount Healthcare Co-Invest IV L.P.

The filing states this Amendment No. 2 is being made solely to disclose a new lock-up agreement tied to the company’s underwritten public offering, and that the number of shares, pre-funded warrants, and preferred shares beneficially owned is unchanged from the prior amendment. In connection with the offering that closed on June 5, 2026, the reporting persons agreed not to sell company securities for 60 days following the date of the final prospectus supplement, unless the underwriters’ representatives consent. They did not purchase securities or otherwise participate in the offering.

The disclosure also notes that 5,790,206 common shares underlying pre-funded warrants and 3,106,000 shares issuable upon conversion of additional Series A preferred shares are excluded from beneficial ownership calculations due to 9.99% and 19.99% beneficial ownership limitations. Separately, stock options held by Tomas Kiselak to acquire 23,017 common shares at an exercise price of $10.14 per share vested and became exercisable on April 29, 2026.

Key Figures

Total beneficial ownership: 14,069,494 shares Beneficial ownership percentage: 19.99% Fund II holdings: 11,413,677 shares +5 more

8 metrics

Total beneficial ownership 14,069,494 shares Reported by Fairmount Funds Management LLC and affiliates; 19.99% of class

Beneficial ownership percentage 19.99% Percent of Jade Biosciences common stock reported as beneficially owned

Fund II holdings 11,413,677 shares Fairmount Healthcare Fund II L.P.; 16.22% of class

Co-Invest holdings 2,655,817 shares Fairmount Healthcare Co-Invest IV L.P.; 4.36% of class

Shares deemed outstanding 70,384,984 shares Common stock outstanding baseline used for Row 13 as of June 5, 2026

Pre-funded warrant overhang 5,790,206 shares Common shares issuable on exercise of pre-funded warrants, subject to 9.99% cap

Series A preferred overhang 3,135,000 shares Common shares issuable from 3,135 Series A Preferred, subject to 19.99% cap

Kiselak option terms 23,017 shares at $10.14 Director stock options that vested and became exercisable on April 29, 2026

Key Terms

Series A non-voting convertible preferred stock, Pre-Funded Warrants, beneficial ownership limitation, Lock-Up Agreement, +1 more

5 terms

Series A non-voting convertible preferred stock financial

"9,516,000 shares of Common Stock issuable upon conversion of 9,516 shares of Series A non-voting convertible preferred stock"

Series A non-voting convertible preferred stock is an early-round ownership share that gives holders priority over common shareholders for payouts and protections, but does not grant voting control. It can be exchanged later for common shares—like a coupon that can be turned into regular stock—allowing investors to share in upside while limiting immediate influence on company decisions; this affects potential returns, dilution for other shareholders, and the balance of control in future financing or sale events.

Pre-Funded Warrants financial

"5,790,206 shares of Common Stock issuable upon exercise of Pre-Funded Warrants"

Pre-funded warrants are financial instruments that give investors the right to purchase a company's stock at a set price, but with most or all of the purchase price paid upfront. They function like a coupon or gift card for stock, allowing investors to buy shares later at a fixed price, which can be beneficial if they want to avoid future price increases. This makes them important for investors seeking flexibility and certainty in their investment plans.

beneficial ownership limitation financial

"The exercise of the Pre-Funded Warrants is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock"

A beneficial ownership limitation is a rule that caps the percentage of a company’s shares an investor can be treated as owning or controlling for voting, regulatory or tax purposes. It matters to investors because it can restrict how many shares a person or group can buy or vote, affect takeover chances, and influence share liquidity and value — like a speed limit that prevents any single driver from taking over the whole road.

Lock-Up Agreement financial

"the Reporting Persons entered into a customary lock-up letter agreement (the "Lock-Up Agreement")"

A lock-up agreement is a contract that prevents company insiders and early investors from selling their shares for a fixed period after a stock sale, often after an initial public offering. It matters to investors because it temporarily limits the number of shares that can hit the market, which can keep the share price steadier; when the lock-up ends, a sudden increase in available shares can create extra volatility, revealing insiders’ confidence or lack thereof.

underwritten public offering financial

"In connection with the Company's underwritten public offering of common stock that closed on June 5, 2026"

An underwritten public offering is when a company sells new shares of its stock to the public with the help of a financial firm, called an underwriter. The underwriter agrees to buy all the shares upfront, reducing the company's risk, and then sells them to investors. This process helps companies raise money quickly and confidently from a wide range of buyers.

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Available on EDGAR 06/09/2026 - 06:51 PM Accepted by SEC EDGAR 06/09/2026 - 05:30 PM Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 2)

Jade Biosciences, Inc.

(Name of Issuer)

Common Stock, $0.0001 par value per share

(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)

06/05/2026

(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).

schemaVersion:

SCHEDULE 13D

CUSIP Number(s): 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 14,069,494.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 14,069,494.00

11

Aggregate amount beneficially owned by each reporting person

14,069,494.00

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") and 9,516,000 shares of Common Stock issuable upon conversion of 9,516 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) 5,790,206 shares of Common Stock issuable upon exercise of Pre-Funded Warrants and (ii) 3,106,000 shares of Common Stock issuable upon conversion of 3,106 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 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 70,384,984 shares of Common Stock outstanding as of June 5, 2026, consisting of (i) 60,845,967 shares of Common Stock outstanding as of June 5, 2026, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated June 3, 2026, (ii) 23,017 shares underlying options that are currently exercisable or will be exercisable within 60 days of the date of this filing by the Reporting Persons and (iii) the 9,516,000 shares of Common Stock underlying the 9,516 shares of Series A Preferred Stock owned by the Reporting Persons, subject to the beneficial ownership limitation.

SCHEDULE 13D

CUSIP Number(s): 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 11,413,677.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 11,413,677.00

11

Aggregate amount beneficially owned by each reporting person

11,413,677.00

12

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

13

Percent of class represented by amount in Row (11)

16.22 %

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 and 9,516,000 shares of Common Stock issuable upon conversion of 9,516 Series A Preferred Stock held directly by Fund II, and (b) exclude 5,790,206 shares of Common Stock issuable upon exercise of Pre-Funded Warrants and 3,106,000 shares of Common Stock issuable upon conversion of 3,106 shares of Series A Preferred Stock. 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%. Row 13 is based on 70,384,984 shares of Common Stock outstanding as of June 5, 2026, consisting of (i) 60,845,967 shares of Common Stock outstanding as of June 5, 2026, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated June 3, 2026, (ii) 23,017 shares underlying options that are currently exercisable or will be exercisable within 60 days of the date of this filing by the Reporting Persons and (iii) the 9,516,000 shares of Common Stock underlying the 9,516 shares of Series A Preferred Stock owned by the Reporting Persons, subject to the beneficial ownership limitation.

SCHEDULE 13D

CUSIP Number(s): 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)

4.36 %

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 60,845,967 shares of Common Stock outstanding as of June 5, 2026, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated June 3, 2026.

SCHEDULE 13D

CUSIP Number(s): 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 14,069,494.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 14,069,494.00

11

Aggregate amount beneficially owned by each reporting person

14,069,494.00

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 Number(s): 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 23,017.00
8	Shared Voting Power 14,040,494.00
9	Sole Dispositive Power 23,017.00
10	Shared Dispositive Power 14,040,494.00

11

Aggregate amount beneficially owned by each reporting person

14,063,511.00

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 securities include (a) 23,017 shares of Common Stock issuable upon the exercise of options that are currently exercisable or will be exercisable within 60 days of the date of this filing held directly by Mr. Kiselak*, (b) Fund II's direct holdings of (i) 1,897,677 shares of Common Stock and (ii) 9,487,000 shares of Common Stock issuable upon conversion of 9,487 shares of Series A Preferred Stock held directly by Fund II, and (c) Co-Invest's direct holdings of 2,655,817 shares of Common Stock. The securities exclude (i) 5,790,206 shares of Common Stock issuable upon exercise of Pre-Funded Warrants and (ii) 3,135,000 shares of Common Stock issuable upon conversion of 3,135 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%. 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 70,355,984 shares of Common Stock outstanding as of June 5, 2026, consisting of (i) 60,845,967 shares of Common Stock outstanding as of June 5, 2026, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated June 3, 2026, (ii) 23,017 shares underlying options that are currently exercisable or will be exercisable within 60 days of the date of this filing by the Reporting Persons and (iii) the 9,487,000 shares of Common Stock underlying the 9,487 shares of Series A Preferred Stock owned by the Reporting Persons, subject to the beneficial ownership limitation. * Under Mr. Kiselak's arrangement with Fairmount, Mr. Kiselak holds the options for one or more investment vehicles managed by Fairmount (each, a "Fairmount Fund"). Mr. Kiselak is obligated to turn over to Fairmount any net cash or stock received from the option for the benefit of such Fairmount Fund. Mr. Kiselak therefore disclaims beneficial ownership of the option and underlying common stock.

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: 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. 2 amends and supplements the statement on Schedule 13D originally filed with the Securities and Exchange Commission (the "SEC") on May 1, 2025 and Amendment No. 1 on October 8, 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. 2 shall have the meanings ascribed to them in the Statement. Unless amended or supplemented below, the information in the Statement remains unchanged. This Amendment No. 2 is being filed solely to report the Reporting Persons' entry into a lock-agreement with the Company in connection with the Company's underwritten public offering; the number of shares of Common Stock, Pre-Funded Warrants, and Series A Preferred Stock beneficially owned by the Reporting Persons has not changed since Amendment No. 1 to the Statement, filed on October 8, 2025.
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. 2 is incorporated by reference herein. The Reporting Persons' securities (a) include (i) 1,897,677 shares of Common Stock and 9,487,000 shares of Common Stock issuable upon conversion of 9,487 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) 5,790,206 shares of Common Stock issuable upon exercise of Pre-Funded Warrants and (ii) 3,135,000 shares of Common Stock issuable upon conversion of 3,135 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%.
(b)	The information in rows 7 through 10 of each of the cover pages of this Amendment No. 2 is incorporated by reference herein.
(c)	On April 29, 2026, Mr. Kiselak's stock options to purchase 23,017 shares of Common Stock at an exercise price of $10.14 per share, received as compensation for his service as a director of the Company, vested in full and became exercisable.
Item 6.	Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer
	Lock-Up Agreement In connection with the Company's underwritten public offering of common stock that closed on June 5, 2026, the Reporting Persons entered into a customary lock-up letter agreement (the "Lock-Up Agreement") with Jefferies LLC, TD Securities (USA) LLC, and UBS Securities LLC, acting as the representatives of the underwriters (collectively, the "Representatives") to an underwriting agreement with the Company. Pursuant to the Lock-Up Agreement, the Reporting Persons agreed to refrain from selling shares of the Company's securities without the consent of the Representatives, and other customary lock-up conditions, for a period of 60 days following the date of the final prospectus supplement for the offering. The Reporting Persons did not purchase any securities of the Company or otherwise participate in the offering. The foregoing description of the Lock-Up Agreement does not purport to be complete and is qualified in its entirety by reference to the Form of Lock-Up Agreement, which is filed as Exhibit 99.2 to this Schedule 13D and incorporated herein by reference.
Item 7.	Material to be Filed as Exhibits.
	99.1 Joint Filing Agreement (incorporated by reference to Exhibit 99.1 of the Reporting Persons' Schedule 13D filed with the SEC on October 8, 2025). 99.2 Lock-Up Agreement (incorporated by reference to Exhibit A to the Underwriting Agreement filed as Exhibit 1.1 of the Company's Form 8-K filed with the SEC on June 4, 2026).

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:	06/09/2026

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	06/09/2026

Fairmount Healthcare Fund II L.P.

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin, Managing Member
Date:	06/09/2026

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	06/09/2026

Fairmount Healthcare Co-Invest IV L.P.

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin, Managing Member
Date:	06/09/2026

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak, Managing Member
Date:	06/09/2026

Peter Evan Harwin

Signature:	/s/ Peter Harwin
Name/Title:	Peter Harwin
Date:	06/09/2026

Tomas Kiselak

Signature:	/s/ Tomas Kiselak
Name/Title:	Tomas Kiselak
Date:	06/09/2026

FAQ

What percentage of Jade Biosciences (JBIO) does Fairmount currently report owning?

Fairmount Funds and affiliates report beneficial ownership of 14,069,494 shares of Jade Biosciences common stock, representing 19.99% of the outstanding class. This figure reflects common shares plus certain convertible preferred shares, calculated against 70,384,984 shares deemed outstanding as of June 5, 2026.

How is Fairmount’s Jade Biosciences (JBIO) stake split between its funds?

Fairmount Healthcare Fund II L.P. reports 11,413,677 shares beneficially owned, or 16.22% of the class, including common and convertible preferred shares. Fairmount Healthcare Co-Invest IV L.P. holds 2,655,817 common shares, representing 4.36% of the outstanding Jade Biosciences common stock as of June 5, 2026.

What lock-up terms did Fairmount agree to regarding Jade Biosciences (JBIO)?

In connection with Jade Biosciences’ underwritten public offering that closed June 5, 2026, the reporting persons signed a 60-day lock-up agreement. They agreed not to sell company securities without underwriter consent for 60 days after the date of the final prospectus supplement for the offering.

Did Fairmount or its affiliates buy new Jade Biosciences (JBIO) shares in the recent offering?

No. The filing states the reporting persons did not purchase any securities of Jade Biosciences and did not otherwise participate in the underwritten public offering that closed on June 5, 2026. Their reported beneficial ownership levels are unchanged from the prior amendment.

What are the beneficial ownership limitations on Fairmount’s Jade Biosciences (JBIO) warrants and preferred stock?

Pre-funded warrants held by Fairmount are subject to a 9.99% beneficial ownership cap, and Series A preferred stock conversions are capped at 19.99%. Shares issuable above these thresholds are excluded from current beneficial ownership. If Fairmount’s group falls to 9.0% or less, the preferred cap drops to 9.99%.

What equity compensation related to Jade Biosciences (JBIO) does Tomas Kiselak hold?

On April 29, 2026, stock options held by Tomas Kiselak to buy 23,017 shares of Jade Biosciences common stock at an exercise price of $10.14 per share vested and became exercisable. The filing notes he holds these options for Fairmount-managed investment vehicles and disclaims beneficial ownership.