As with ETV6/RUNX1-rearranged and TCF3/PBX1-rearranged B-ALL, a study in monozygotic twins showed that TCF3/ZNF384 fusion can occur in utero, suggesting that a fetal hematopoietic progenitor is the cell of origin in this ALL subgroup [81]. This evidence concerns the gene PBX1 and acute lymphoblastic leukemia.