These data show that in childhood Ph(+) ALL, BCR-ABL1 gene fusion can be a prenatal and possibly initiating genetic event.     ///    "The twin pair concordant for ALL shared identical BCR-ABL1 genomic sequence indicative of monoclonal, in utero origin. "    ///    Low level BCR-ABL1(+) cells were present in the healthy co-twin but lacked the IKZF1 deletion present in the other twin's leukemic cells.     ///    " The timing and developmental sequence of events for BCR-ABL1(+) acute lymphoblastic leukemia (ALL), usually associated with IKAROS (IKZF1) deletions, are unknown. "    ///    "We assessed the status of BCR-ABL1 and IKZF1 genes in 2 pairs of monozygotic twins, one pair concordant, the other discordant for Philadelphia chromosome positive (Ph(+)) ALL. "    ///    "In the twin pair discordant for ALL, neonatal blood spots from both twins harbored the same clonotypic BCR-ABL1 sequence. "    ///    Developmental origins and impact of BCR-ABL1 fusion and IKZF1 deletions in monozygotic twins with Ph+ acute lymphoblastic leukemia.. 