The correlation between infant leukemia and in utero exposure to topoisomerase II (topo-II) inhibitor has been clarified. We examined the in vitro effect of topo-II inhibitor (etoposide) on cleavage of the MLL gene in cord and peripheral blood mononuclear cells (MNCs). Southern blot analysis showed cleavage of the MLL gene in peripheral blood MNCs of infants when the MNCs were exposed to etoposide. MNCs were incubated with etoposide at various concentrations (1 to 50 microM), and a ligation-mediated polymerase chain reaction (LM-PCR) was used to detect double strand breaks (DSBs) of DNA in intron 8 of the MLL breakpoint cluster region. PCR products obtained with LM-PCR were subcloned and sequenced to identify the breakpoint in the MLL gene. The PCR products indicated DSBs of the MLL gene were obtained without any difference in the incidence between 3 different samples (cord and peripheral blood from infants and children). Sequencing analysis showed that the DSBs occurred on the telomeric side of intron 8 and near exon 9. There was no evidence that the cord blood was more susceptible to MLL DNA breakage by topo-II inhibitor than were other cells. Instability of the partner gene during the fetal period could be associated with the pathogenesis of infant leukemia.