Autologous chimeric antigen receptor (CAR)-modified T cells that target the B cell antigen CD19 and that express the CD137 (a costimulatory receptor) signalling domain have been used in adults with chronic lymphocytic leukaemia (CLL). Grupp et al. tested these cells in two children with relapsed and refractory pre-B cell acute lymphoblastic leukaemia (ALL). Although several adverse events occurred, both patients had a complete remission, and this is still ongoing in one patient after 11 months. The other patient relapsed after 2 months, with cells that no longer expressed CD19, indicating that other molecules may need to be targeted in some patients.