Recently, RNA in situ hybridization (RNA ISH) approaches have been successfully applied to track CAR T cells in patients with glioblastoma4, in a case study in a patient with B-ALL, who died 5 days post-anti-CD19 CAR T cell infusion5 and in clinical samples from the ZUMA-1 trial that tested the anti-CD19 CAR Axicabtagene ciloleucel6. Here, CD19 is linked to acute lymphoblastic leukemia.