The HMT mixed-lineage leukaemia (MLL, also known as lysine methyltransferase 2A (KMT2A)) has the ability to catalyse mono-, di-, and, to a lesser extent, trimethylation of histone H3K4 [19,20,21,22,23], and plays a pivotal role in haematological malignancies, as described below. The gene discussed is KMT2A; the disease is leukemia.