The lysine methyltransferase 2A (KMT2A) gene rearrangement, formerly known as the mixed lineage leukemia (MLL) gene or HRX rearrangement, occurs in 70–75% of infants, 5–6% of elderly children or 10–15% of adult patients with precursor B cell acute lymphoblastic leukemia (B-ALL) [1–3]. This evidence concerns the gene KMT2A and precursor B-cell acute lymphoblastic leukemia.