Rearrangements of the Mixed Lineage Leukemia (MLL, also known as KMT2A) gene (MLLr) cause aggressive, poor prognosis acute lymphoblastic (ALL) and acute myeloid (AML) leukemias in both children and adults29–31 and are associated with very few cooperating mutations32–36. The gene discussed is KMT2A; the disease is leukemia.