X0202 SCHEDULE 13G 0001931056 XXXXXXXX LIVE Class A ordinary shares, par value $0.0001 per share 10/19/2026 0002149916 TCGX Acquisition Corp. II G87082106 245 Lytton Ave. Suite 350 Palo Alto CA 94301 Rule 13d-1(c) Third Rock Ventures VI, L.P. b DE 0 990000 0 990000 990000 N 9.5 PN Third Rock Ventures GP VI, L.P. b DE 0 990000 0 990000 990000 N 9.5 PN TRV GP VI, LLC b DE 0 990000 0 990000 990000 N 9.5 OO TCGX Acquisition Corp. II 245 Lytton Ave., Suite 350, Palo Alto, CA 94301 (i) Third Rock Ventures VI, L.P. ("TRV VI"); (ii) Third Rock Ventures GP VI, L.P. ("TRV GP VI"), which is the sole general partner of TRV VI; and (iii) TRV GP VI, LLC ("TRV GP VI LLC"), which is the sole general partner of TRV GP VI. The address of the principal business office of each of the Reporting Persons is Third Rock Ventures, LLC, 201 Brookline Ave, Suite 1401, Boston, MA 02215. Each of TRV VI and TRV GP VI is a Delaware limited partnership. TRV GP VI LLC is a Delaware limited liability company. Y (i) TRV VI directly owns 990,000 Class A ordinary shares (the "VI Shares"). (ii) TRV GP VI is the general partner of TRV VI and may be deemed to beneficially own the VI Shares. (iii) TRV GP VI LLC is the general partner of TRV GP VI and may be deemed to beneficially own the VI Shares. Each of TRV VI, TRV GP VI and TRV GP VI LLC disclaim the existence of a "group" for purposes of Rule 13d-5 of the Securities Exchange Act of 1934, as amended, and nothing contained in this report shall be deemed an admission that any such group exists or may exist. The VI Shares represent approximately 9.5% of the outstanding Class A ordinary shares. The percent of class was based upon 10,450,000 Class A ordinary shares outstanding following the closing of the Issuer's initial public offering and concurrent private placement on October 19, 2026, as disclosed in the Issuer's final prospectus filed with the Securities and Exchange Commission pursuant to Rule 424(b)(4) on October 16, 2026. TRV VI - 0 TRV GP VI - 0 TRV GP VI LLC - 0 TRV VI - 990,000 TRV GP VI - 990,000 TRV GP VI LLC - 990,000 TRV VI - 0 TRV GP VI - 0 TRV GP VI LLC - 0 TRV VI - 990,000 TRV GP VI - 990,000 TRV GP VI LLC - 990,000 Y Y Y Y Y N 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. Third Rock Ventures VI, L.P. /s/ Kevin Gillis 10/26/2026 Third Rock Ventures GP VI, L.P. /s/ Kevin Gillis 10/26/2026 TRV GP VI, LLC /s/ Kevin Gillis 10/26/2026