The chromosomal translocation occurring between the band 21 of the long arm of chromosome 4 and band 23 of the long arm of chromosome 11 [t(4;11)(q21;q23)] and leading to the generation of the fusion gene MLL-AF4 is one of the most recurrent chromosomal aberrations observed in acute lymphoblastic leukemia (ALL). The gene discussed is KMT2A; the disease is acute lymphoblastic leukemia.