LS used to be a very rare phenomenon, accounting for less than 1% of leukemia cases1, but now presents as a serious complication in the context of emerging CD19 targeted immunotherapies such as bispecific T-cell engager, blinatumomab, and chimeric antigen receptor T cells (CAR-T cells); it is particularly problematic in the context of KMT2A(MLL) rearranged acute lymphoblastic leukemia (KMT2A-r ALL)2–5. The gene discussed is KMT2A; the disease is acute lymphoblastic leukemia.