RA Capital Management and related reporting persons report beneficial ownership of 500,000 ordinary shares (6.4%) of JATT II Acquisition Corp. The filing states the Fund directly holds 500,000 ordinary shares and that the 6.4% percentage is based on 7,800,000 ordinary shares outstanding as of April 20, 2026, per the issuer's final prospectus dated April 17, 2026. The filing describes voting and dispositive power relationships: the Fund delegated sole voting and dispositive power to RA Capital, and RA Capital's managers (Peter Kolchinsky and Rajeev Shah) are identified as controlling persons. The Reporting Persons disclaim group status and disclaim beneficial ownership except for Section 13(d) reporting purposes.
Positive
None.
Negative
None.
Insights
Blocks of affiliated holders report a 6.4% stake via delegated voting and disposition.
The filing documents a single, clearly quantified holding: the Fund directly holds 500,000 shares, reported as 6.4% of 7,800,000 outstanding shares as of April 20, 2026. RA Capital is the adviser with delegated voting/dispositive authority, and two named managers are identified as controlling persons.
The practical implications depend on whether the Fund or RA Capital chooses to exercise that delegated authority; subsequent transaction activity would appear in later ownership filings. Cash‑flow treatment and any planned dispositions are not described in the excerpt.
Key Figures
Shares held by Fund:500,000 sharesPercent of class:6.4%Shares outstanding (context):7,800,000 ordinary shares+1 more
4 metrics
Shares held by Fund500,000 sharesDirectly held by RA Capital Healthcare Fund, L.P.
Percent of class6.4%Based on 7,800,000 ordinary shares outstanding as of April 20, 2026
Shares outstanding (context)7,800,000 ordinary sharesReported in issuer's final prospectus dated April 17, 2026; as of April 20, 2026
CUSIPG50765109Issuer CUSIP for ordinary shares
Key Terms
Schedule 13G, beneficial ownership, shared dispositive power, delegated voting power
4 terms
Schedule 13Gregulatory
"The form_type is SCHEDULE 13G and the Reporting Persons disclaim group status"
A Schedule 13G is a formal document that investors file with the government when they acquire a large ownership stake in a company, usually for investment purposes rather than control. It helps keep the public informed about who owns significant parts of a company's shares, which can influence how the company is managed and how investors make decisions. Filing this schedule is important for transparency and understanding the ownership landscape of publicly traded companies.
beneficial ownershipfinancial
"Amount beneficially owned: Row 9 of each Reporting Person's cover page"
Beneficial ownership means the person or entity that actually enjoys the benefits of owning shares or other assets — such as receiving dividends, voting rights, or price gains — even if the legal title is held in another name. For investors it matters because knowing who truly controls and profits from a company reveals who can influence decisions, exposes potential conflicts of interest or hidden concentration of power, and affects transparency and risk in the stock.
shared dispositive powerregulatory
"Shared Dispositive Power 500,000.00 on each cover page row"
delegated voting powerregulatory
"The Fund has delegated to RA Capital the sole power to vote and the sole power to dispose"
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SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
UNDER THE SECURITIES EXCHANGE ACT OF 1934
JATT II Acquisition Corp
(Name of Issuer)
Ordinary Shares, par value $0.0001 per share
(Title of Class of Securities)
G50765109
(CUSIP Number)
04/20/2026
(Date of Event Which Requires Filing of this Statement)
Check the appropriate box to designate the rule pursuant to which this Schedule is filed:
Rule 13d-1(b)
Rule 13d-1(c)
Rule 13d-1(d)
schemaVersion:
SCHEDULE 13G
CUSIP Number(s):
G50765109
1
Names of Reporting Persons
RA Capital Management, 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
500,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
500,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
500,000.00
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)
6.4 %
12
Type of Reporting Person (See Instructions)
IA, PN
SCHEDULE 13G
CUSIP Number(s):
G50765109
1
Names of Reporting Persons
Peter Kolchinsky
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
500,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
500,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
500,000.00
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)
6.4 %
12
Type of Reporting Person (See Instructions)
HC, IN
SCHEDULE 13G
CUSIP Number(s):
G50765109
1
Names of Reporting Persons
Rajeev Shah
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
500,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
500,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
500,000.00
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)
6.4 %
12
Type of Reporting Person (See Instructions)
HC, IN
SCHEDULE 13G
CUSIP Number(s):
G50765109
1
Names of Reporting Persons
RA Capital Healthcare 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
500,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
500,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
500,000.00
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)
6.4 %
12
Type of Reporting Person (See Instructions)
PN
SCHEDULE 13G
Item 1.
(a)
Name of issuer:
JATT II Acquisition Corp
(b)
Address of issuer's principal executive offices:
153 Central Avenue, C/O 56, Westfield, NJ, 07091.
Item 2.
(a)
Name of person filing:
The names of the persons filing this report (collectively, the "Reporting Persons") are:
RA Capital Management, L.P. ("RA Capital")
Peter Kolchinsky ("Dr. Kolchinsky")
Rajeev Shah ("Mr. Shah")
RA Capital Healthcare Fund, L.P. (the "Fund")
The Reporting Persons expressly disclaim status as a "group" for purposes of this Schedule 13G.
(b)
Address or principal business office or, if none, residence:
The address of the principal business office of each of the Reporting Persons is:
c/o RA Capital Management, L.P., 200 Berkeley Street, 18th Floor, Boston MA 02116
(c)
Citizenship:
RA Capital and the Fund are Delaware limited partnerships.
Dr. Kolchinsky and Mr. Shah are United States citizens.
(d)
Title of class of securities:
Ordinary Shares, par value $0.0001 per share
(e)
CUSIP Number(s):
G50765109
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:
Row 9 of each Reporting Person's cover page to this Schedule 13G sets forth the aggregate number of ordinary shares of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.
The Fund directly holds 500,000 ordinary shares.
RA Capital Healthcare Fund GP, LLC is the general partner of the Fund. The general partner of RA Capital is RA Capital Management GP, LLC, of which Dr. Kolchinsky and Mr. Shah are the controlling persons. RA Capital serves as investment adviser for the Fund and may be deemed a beneficial owner, for purposes of Section 13(d) of the Act, of any securities of the Issuer held by the Fund. The Fund has delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in the Fund's portfolio, including the ordinary shares of the Issuer reported herein. Because the Fund has divested voting and investment power over the reported securities it holds and may not revoke that delegation on less than 61 days' notice, the Fund disclaims beneficial ownership of the securities it holds for purposes of Section 13(d) of the Act. As managers of RA Capital, Dr. Kolchinsky and Mr. Shah may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Issuer beneficially owned by RA Capital. RA Capital, Dr. Kolchinsky, and Mr. Shah disclaim beneficial ownership of the securities reported in this Schedule 13G other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13G shall not be deemed an admission that either RA Capital, Dr. Kolchinsky, or Mr. Shah is the beneficial owner of such securities for any other purpose.
(b)
Percent of class:
Row 11 of each Reporting Person's cover page to this Schedule 13G sets forth the percentages of ordinary shares of the Issuer beneficially owned by such Reporting Person and is incorporated by reference. Such percentage is based upon 7,800,000 ordinary shares outstanding as of April 20, 2026, as reported in the Issuer's final prospectus dated April 17, 2026 filed with the Securities and Exchange Commission (the "SEC") on April 17, 2026.
(c)
Number of shares as to which the person has:
(i) Sole power to vote or to direct the vote:
Row 5 of each Reporting Person's cover page to this Schedule 13G sets forth the sole power to vote or to direct the vote of securities of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.
(ii) Shared power to vote or to direct the vote:
Row 6 of each Reporting Person's cover page to this Schedule 13G sets forth the shared power to vote or to direct the vote of securities of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.
(iii) Sole power to dispose or to direct the disposition of:
Row 7 of each Reporting Person's cover page to this Schedule 13G sets forth the sole power to dispose or to direct the disposition of securities of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.
(iv) Shared power to dispose or to direct the disposition of:
Row 8 of each Reporting Person's cover page to this Schedule 13G sets forth the shared power to dispose or to direct the disposition of securities of the Issuer beneficially owned by such Reporting Person and is incorporated by reference.
Item 5.
Ownership of 5 Percent or Less of a Class.
Not Applicable
Item 6.
Ownership of more than 5 Percent on Behalf of Another Person.
Not Applicable
Item 7.
Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
Not Applicable
Item 8.
Identification and Classification of Members of the Group.
Not Applicable
Item 9.
Notice of Dissolution of Group.
Not Applicable
Item 10.
Certifications:
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.
RA Capital Management, L.P.
Signature:
/s/ Peter Kolchinsky
Name/Title:
By Peter Kolchinsky, Authorized Signatory
Date:
04/27/2026
Peter Kolchinsky
Signature:
/s/ Peter Kolchinsky
Name/Title:
Peter Kolchinsky
Date:
04/27/2026
Rajeev Shah
Signature:
/s/ Rajeev Shah
Name/Title:
Rajeev Shah
Date:
04/27/2026
RA Capital Healthcare Fund, L.P.
Signature:
/s/ Peter Kolchinsky
Name/Title:
By RA Capital Healthcare Fund GP, LLC, its General Partner, By Peter Kolchinsky, Manager
What stake does RA Capital hold in JATT II Acquisition Corp (JATT)?
RA Capital and affiliated reporting persons report beneficial ownership of 500,000 shares, representing 6.4% of the class. This percentage uses 7,800,000 ordinary shares outstanding as of April 20, 2026 from the issuer's prospectus.
Who specifically holds the 500,000 shares reported in the filing?
The filing states the RA Capital Healthcare Fund, L.P. directly holds 500,000 ordinary shares. RA Capital serves as investment adviser and has delegated voting and dispositive power over the Fund's portfolio holdings.
Do Peter Kolchinsky and Rajeev Shah directly control the reported shares?
The filing names Peter Kolchinsky and Rajeev Shah as controlling persons of RA Capital. They may be deemed beneficial owners for Section 13(d) purposes, but they and RA Capital disclaim beneficial ownership except for reporting obligations.
Is this filing a Schedule 13G or a Schedule 13D and what does that mean?
This is a Schedule 13G beneficial ownership filing. It reports passive or qualified institutional holdings under Section 13(g) reporting procedures, with the Reporting Persons expressly disclaiming group status in the excerpt.
What voting and dispositive powers are reported for these holders?
Cover‑page rows incorporated by reference show 0 sole voting power and 500,000 shared voting power, and similarly 0 sole dispositive and 500,000 shared dispositive power for each Reporting Person, per the filing language.
