CD28 and acute lymphoblastic leukemia: Different malignancies might benefit from different CAR T cell costimulatory domains: while the longer persistence of CAR T cells employing 4‐1BB costimulatory domains may be important for long‐term remission of the precursor B‐cell malignancy B‐ALL, long‐term CAR T cell persistence may be less critical than early antitumor activity when treating mature B‐NHL malignancies.37 However, variability in CAR design, in vector, in manufacturing processes and between patient cohorts precludes definitive comparisons across CD28 and 4‐1BB‐based CAR T cell clinical trials.