Gardner et al. reported on 2 of 7 patients with B-ALL harboring rearrangement of the mixed lineage leukemia (MLL) gene and achieving molecular CR after anti-CD19 CAR-T cell infusion developing AML that was clonally related to their B-ALL within 1 month after anti-CD19 CAR-T cell infusion [41]. The gene discussed is KMT2A; the disease is acute lymphoblastic leukemia.