"Infant pro-B acute lymphoblastic leukemia (ALL) carrying the fusion oncogene MLL-AF4 is associated with very brief latency and dismal prognosis. Recent studies have revealed an in utero origin of MLL-AF4 but the nature of the target cell for transformation in the embryo/fetus is unknown. Because of the mixed phenotype and the presence of MLL-AF4 in both lymphoid and myeloid lineages, hematopoieti ...