Human mixed lineage leukemia protein-1 (MLL1, KMT2A, EC 2.1.1.43) is a member of the SET1 family of histone H3 lysine 4 (H3K4) methyltransferases and is frequently altered in acute myeloid and lymphoid leukemias (1–4). Here, KMT2A is linked to lymphoid leukemia.