MLL1 (KMT2A) is a component of the COMPASS complex, and MLL fusion proteins bind to the HOXA locus in AML cells and recruit the non-COMPASS histone H3 methyltransferase DOT1L, which is required for HOXA activation and AML development in MLL-rearranged leukemia models [4–7]. The gene discussed is KMT2A; the disease is acute myeloid leukemia.