Disclosure forms provided by the authors are available with the full text of this article at NEJM.org.

A data sharing statement provided by the authors is available with the full text of this article at NEJM.org.

We thank the patients and their families and caregivers; the trial teams at the participating sites; Manuel Rodrigues, M.D., Ph.D., Christophe Le Tourneau, M.D., Ph.D., Heather Shaw, M.D., Antje Blank, M.D., Monika Dudzisz-Sledz, M.D., Ph.D., Frank Cornélis, M.D., Filomena Mazzeo, M.D., David Hogg, M.D., Anna Spreafico, M.D., Ph.D., and Sam Saibil, M.D., Ph.D., for the provision of trial materials, referral of patients, patient care, or a combination of these; Igor Puzanov, M.D., Serge Leyvraz, M.D., Takami Sato, M.D., Ph.D., and Mark R. Middleton, M.D., Ph.D., for their insights and guidance regarding the tebentafusp development program; Ramakrishna Edukulla, Ph.D., of Immunocore, for statistical analysis support; Michelle L. McCully, Ph.D., of Immunocore, for writing and editorial assistance; and David Berman, M.D., Ph.D., and Mohammed Dar, M.D., both of Immunocore, for their leadership and critical review of an earlier version of the manuscript.