TCF3 and leukemia: Studies on monozygotic twins with concordant leukemia and retrospective analyses of neonatal blood spots (Guthrie Card) of pediatric patients proved the frequent prenatal origin of some of these genetic aberrations, such as hyperdiploidy, KMT2A rearrangements, BCR-ABL1, TCF3-PBX1, or ETV6-RUNX1. The concordance of disease in twins has been explained by the generation of the genetic alteration in utero in one fetus, followed by intraplacental metastasis of the clonal progeny to the other twin.