Consistent with a role for FOXO1 in maintaining CAR T cells in a favourable differentiation state, deletion of FOXO1 in mouse CAR T cells led to a significant reduction in CD62L and TCF7 expression, and upon serial co-culture with E0771-HER2 tumour cells led to significantly increased expression of TIM3, PD-1 and LAG3, altogether indicative of a transition to a more short-term effector-like phenotype (Extended Data Fig. 1k–m). Here, SELL is linked to neoplasm.