The first study published on this topic examined three pairs of identical infant twins with KMT2A-rearranged ALL and showed that the leukemia cells of each twin pair harbored KMT2A rearrangements that were identical within the pair but distinct from the other twin pairs as well as KMT2A rearrangements observed in eight infant ALL cases from singleton births. The gene discussed is KMT2A; the disease is acute lymphoblastic leukemia.