Rearrangements of the Mixed Lineage Leukemia (MLL) gene generated by reciprocal translocations involving chromosome band 11q23 are well-known in infants and adults with acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL), as well as in 85% of secondary leukemias associated with a history of treatment with topoisomerase II inhibitors (Adler et al., 1999; Ross, 2000). This evidence concerns the gene KMT2A and acute lymphoblastic leukemia.