Recurring chromosomal translocations involving chromosome band 11q23 have been observed in acute lymphocytic leukemia (ALL) and acute myeloid leukemia (AML), especially AML with FAB M4 or M5 phenotype. Though numerous partner chromosomes have been documented, the t(4;11) is the translocation seen most commonly in infant ALL. t(4;11) leukemia, associated with hyperleukocytosis, hepatosplenomegaly, and central nervous system (CNS) disease, has a dismal prognosis. Leukemia with 11q23 rearrangement often shows both lymphoid and myeloid characteristics, leading to speculation that the disrupted gene is involved in lymphoid and myeloid differentiation. The genes at 11q23 and 4q21 have been cloned and sequenced; the data is consistent with a role for these genes in transcriptional regulation. Absence of molecular rearrangement of 11q23 identifies a group of infants with a good prognosis.