X4 Pharmaceuticals, Inc. received an amended Schedule 13G/A from Biotechnology Value Fund–affiliated entities and Mark N. Lampert reporting a significant but sub‑5% passive stake. As of December 31, 2025, the group may be deemed to beneficially own 3,411,993 common shares, or about 3.9% of the company.
The position includes Pre-Funded Common Stock Purchase Warrants exercisable for 1,178,251 shares, with a token exercise price of $0.001 per share and no expiry, subject to a 9.99% ownership cap. The filers certify the securities are not held to change or influence control of X4 Pharmaceuticals.
Positive
None.
Negative
None.
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
UNDER THE SECURITIES EXCHANGE ACT OF 1934
(Amendment No. 1)
X4 Pharmaceuticals, Inc
(Name of Issuer)
Common Stock, par value $0.001 per share
(Title of Class of Securities)
98420X202
(CUSIP Number)
12/31/2025
(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)
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
BIOTECHNOLOGY VALUE FUND 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
1,827,005.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
1,827,005.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
1,827,005.00
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)
2.1 %
12
Type of Reporting Person (See Instructions)
PN
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
BVF I GP 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
1,827,005.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
1,827,005.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
1,827,005.00
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)
2.1 %
12
Type of Reporting Person (See Instructions)
OO
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
BIOTECHNOLOGY VALUE FUND II LP
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
1,339,509.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
1,339,509.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
1,339,509.00
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)
1.5 %
12
Type of Reporting Person (See Instructions)
PN
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
BVF II GP 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
1,339,509.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
1,339,509.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
1,339,509.00
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)
1.5 %
12
Type of Reporting Person (See Instructions)
OO
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
Biotechnology Value Trading Fund OS LP
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
CAYMAN ISLANDS
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
194,535.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
194,535.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
194,535.00
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)
0.2 %
12
Type of Reporting Person (See Instructions)
PN
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
BVF Partners OS Ltd.
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
CAYMAN ISLANDS
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
194,535.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
194,535.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
194,535.00
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)
0.2 %
12
Type of Reporting Person (See Instructions)
CO
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
BVF GP HOLDINGS 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
3,166,514.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
3,166,514.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
3,166,514.00
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)
3.6 %
12
Type of Reporting Person (See Instructions)
OO
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
BVF PARTNERS L P/IL
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
3,411,993.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
3,411,993.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
3,411,993.00
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)
3.9 %
12
Type of Reporting Person (See Instructions)
IA, PN
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
BVF INC/IL
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
3,411,993.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
3,411,993.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
3,411,993.00
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)
3.9 %
12
Type of Reporting Person (See Instructions)
CO
SCHEDULE 13G
CUSIP No.
98420X202
1
Names of Reporting Persons
LAMPERT MARK N
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
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
3,411,993.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
3,411,993.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
3,411,993.00
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)
3.9 %
12
Type of Reporting Person (See Instructions)
IN
SCHEDULE 13G
Item 1.
(a)
Name of issuer:
X4 Pharmaceuticals, Inc
(b)
Address of issuer's principal executive offices:
61 North Beacon Street, 4th Floor, Boston, Massachusetts 02134
Item 2.
(a)
Name of person filing:
Biotechnology Value Fund, L.P. ("BVF")
BVF I GP LLC ("BVF GP")
Biotechnology Value Fund II, L.P. ("BVF2")
BVF II GP LLC ("BVF2 GP")
Biotechnology Value Trading Fund OS LP ("Trading Fund OS")
BVF Partners OS Ltd. ("Partners OS")
BVF GP Holdings LLC ("BVF GPH")
BVF Partners L.P. ("Partners")
BVF Inc.
Mark N. Lampert ("Mr. Lampert")
Each of the foregoing is referred to as a "Reporting Person" and collectively as the "Reporting Persons."
(b)
Address or principal business office or, if none, residence:
Biotechnology Value Fund, L.P.
44 Montgomery St., 40th Floor
San Francisco, California 94104
BVF I GP LLC
44 Montgomery St., 40th Floor
San Francisco, California 94104
Biotechnology Value Fund II, L.P.
44 Montgomery St., 40th Floor
San Francisco, California 94104
BVF II GP LLC
44 Montgomery St., 40th Floor
San Francisco, California 94104
Biotechnology Value Trading Fund OS LP
PO Box 309 Ugland House
Grand Cayman, KY1-1104
Cayman Islands
BVF Partners OS Ltd.
PO Box 309 Ugland House
Grand Cayman, KY1-1104
Cayman Islands
BVF GP Holdings LLC
44 Montgomery St., 40th Floor
San Francisco, California 94104
BVF Partners L.P.
44 Montgomery St., 40th Floor
San Francisco, California 94104
BVF Inc.
44 Montgomery St., 40th Floor
San Francisco, California 94104
Mark N. Lampert
44 Montgomery St., 40th Floor
San Francisco, California 94104
(c)
Citizenship:
Biotechnology Value Fund, L.P.
Delaware
BVF I GP LLC
Delaware
Biotechnology Value Fund II, L.P.
Delaware
BVF II GP LLC
Delaware
Biotechnology Value Trading Fund OS LP
Cayman Islands
BVF Partners OS Ltd.
Cayman Islands
BVF GP Holdings LLC
Delaware
BVF Partners L.P.
Delaware
BVF Inc.
Delaware
Mark N. Lampert
United States
(d)
Title of class of securities:
Common Stock, par value $0.001 per share
(e)
CUSIP No.:
98420X202
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:
As of the close of business on December 31, 2025, the Reporting Persons and a certain Partners managed account (the "Partners Managed Account") held Pre-Funded Common Stock Purchase Warrants (the "Pre-Funded Warrants") exercisable for an aggregate of 1,178,251 shares of the Issuer's common stock, par value $0.001 per share (the "Shares"). The Pre-Funded Warrants are exercisable immediately, do not expire, and have an exercise price of $0.001 per Share. The Reporting Persons and the Partners Managed Account (together with their affiliates and other attribution parties) may not exercise any portion of the Pre-Funded Warrants they hold to the extent that, immediately after giving effect to such issuance after exercise, they would beneficially own more than 9.99% of the outstanding Shares (the "Pre-Funded Warrants Blocker"). As of the close of business on December 31, 2025, the Pre-Funded Warrants Blocker did not limit the exercise of any of the Pre-Funded Warrants held by the Reporting Persons or the Partners Managed Account.
As of the close of business on December 31, 2025, (i) BVF beneficially owned 1,827,005 Shares, including 608,395 Shares underlying the Pre-Funded Warrants held by it, (ii) BVF2 beneficially owned 1,339,509 Shares, including 474,169 Shares underlying the Pre-Funded Warrants held by it, and (iii) Trading Fund OS beneficially owned 194,535 Shares, including 71,152 Shares underlying the Pre-Funded Warrants held by it.
BVF GP, as the general partner of BVF, may be deemed to beneficially own the 1,827,005 Shares beneficially owned by BVF.
BVF2 GP, as the general partner of BVF2, may be deemed to beneficially own the 1,339,509 Shares beneficially owned by BVF2.
Partners OS, as the general partner of Trading Fund OS, may be deemed to beneficially own the 194,535 Shares beneficially owned by Trading Fund OS.
BVF GPH, as the sole member of each of BVF GP and BVF2 GP, may be deemed to beneficially own the 3,166,514 Shares beneficially owned in the aggregate by BVF and BVF2.
Partners, as the investment manager of BVF, BVF2 and Trading Fund OS, and the sole member of Partners OS, may be deemed to beneficially own the 3,411,993 Shares beneficially owned in the aggregate by BVF, BVF2 and Trading Fund OS and held in the Partners Managed Account, including 50,944 Shares held in the Partners Managed Account, which includes 24,535 Shares underlying the Pre-Funded Warrants held in the Partners Managed Account.
BVF Inc., as the general partner of Partners, may be deemed to beneficially own the 3,411,993 Shares beneficially owned by Partners.
Mr. Lampert, as a director and officer of BVF Inc., may be deemed to beneficially own the 3,411,993 Shares beneficially owned by BVF Inc.
The foregoing should not be construed in and of itself as an admission by any Reporting Person as to beneficial ownership of any Shares owned by another Reporting Person. BVF GP disclaims beneficial ownership of the Shares beneficially owned by BVF. BVF2 GP disclaims beneficial ownership of the Shares beneficially owned by BVF2. Partners OS disclaims beneficial ownership of the Shares beneficially owned by Trading Fund OS. BVF GPH disclaims beneficial ownership of the Shares beneficially owned by BVF and BVF2. Each of Partners, BVF Inc. and Mr. Lampert disclaims beneficial ownership of the Shares beneficially owned by BVF, BVF2 and Trading Fund OS and held in the Partners Managed Account, and the filing of this statement shall not be construed as an admission that any such person or entity is the beneficial owner of any such securities.
(b)
Percent of class:
The following percentages are based upon a denominator that was calculated as follows: (i) 87,436,688 Shares outstanding as of October 31, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 5, 2025, plus (ii) certain or all of the 1,178,251 Shares underlying the Pre-Funded Warrants held by the Reporting Persons and Partners Managed Account, as applicable.
As of the close of business on December 31, 2025, (i) BVF beneficially owned approximately 2.1% of the outstanding Shares, (ii) BVF2 beneficially owned approximately 1.5% of the outstanding Shares, (iii) Trading Fund OS beneficially owned less than 1% of the outstanding Shares, (iv) BVF GP may be deemed to beneficially own approximately 2.1% of the outstanding Shares, (v) BVF2 GP may be deemed to beneficially own approximately 1.5% of the outstanding Shares, (vi) Partners OS may be deemed to beneficially own less than 1% of the outstanding Shares, (vii) BVF GPH may be deemed to beneficially own approximately 3.6% of the outstanding Shares, and (viii) each of Partners, BVF Inc. and Mr. Lampert may be deemed to beneficially own approximately 3.9% of the outstanding Shares (less than 1% of the outstanding Shares are held in the Partners Managed Account).
(c)
Number of shares as to which the person has:
(i) Sole power to vote or to direct the vote:
See Cover Pages Items 5-9.
(ii) Shared power to vote or to direct the vote:
See Cover Pages Items 5-9.
(iii) Sole power to dispose or to direct the disposition of:
See Cover Pages Items 5-9.
(iv) Shared power to dispose or to direct the disposition of:
See Cover Pages Items 5-9.
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.
If a group has filed this schedule pursuant to §240.13d-1(b)(1)(ii)(J), so indicate under Item 3(j) and attach an exhibit stating the identity and Item 3 classification of each member of the group. If a group has filed this schedule pursuant to §240.13d-1(c) or §240.13d-1(d), attach an exhibit stating the identity of each member of the group.
See Exhibit 99.1 to the Schedule 13G filed by the Reporting Persons with the Securities and Exchange Commission on August 20, 2025.
Item 9.
Notice of Dissolution of Group.
Not Applicable
Item 10.
Certifications:
By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11.
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.
What does the Schedule 13G/A filing for X4 Pharmaceuticals (XFOR) report?
The Schedule 13G/A reports that BVF-affiliated investors and Mark N. Lampert hold a significant but passive stake in X4 Pharmaceuticals. They may be deemed to beneficially own 3,411,993 common shares, representing about 3.9% of the company as of December 31, 2025.
How many X4 Pharmaceuticals shares do the BVF entities and Mark Lampert beneficially own?
They may be deemed to beneficially own 3,411,993 X4 Pharmaceuticals common shares, or approximately 3.9% of the outstanding stock. This total aggregates positions across Biotechnology Value Fund, related funds, a managed account, and entities controlled by BVF Partners and Mark N. Lampert.
What are the key terms of the pre-funded warrants held in X4 Pharmaceuticals (XFOR)?
The group holds Pre-Funded Common Stock Purchase Warrants exercisable for 1,178,251 X4 shares. These pre-funded warrants are exercisable immediately, do not expire, and have a nominal exercise price of $0.001 per share, giving low-cost access to additional equity subject to ownership limits.
What is the 9.99% ownership blocker described in the X4 Pharmaceuticals filing?
The pre-funded warrants include a 9.99% ownership blocker that prevents the holders and related parties from exercising if doing so would push beneficial ownership above 9.99% of outstanding common shares. As of December 31, 2025, this blocker did not restrict exercising any of the warrants.
How was the ownership percentage for X4 Pharmaceuticals (XFOR) calculated in this filing?
Ownership percentages use a base of 87,436,688 common shares outstanding as of October 31, 2025, from X4’s Form 10-Q, plus some or all of 1,178,251 shares underlying the pre-funded warrants. On this denominator, the BVF group’s aggregated 3,411,993 shares equal about 3.9%.
Are the BVF entities and Mark Lampert seeking control of X4 Pharmaceuticals?
The signatory certifies the securities were not acquired and are not held to change or influence control of X4 Pharmaceuticals. The filing is on a Schedule 13G/A, which is used for investors reporting holdings on a passive, non-control-seeking basis under SEC rules.
Which BVF-related entities are included as reporting persons for X4 Pharmaceuticals (XFOR)?
Reporting persons include Biotechnology Value Fund, L.P., Biotechnology Value Fund II, L.P., Biotechnology Value Trading Fund OS LP, various BVF general partner and holding entities, BVF Partners L.P., BVF Inc., and Mark N. Lampert, all tied together through investment management and control relationships.
