Abstract Acute megakaryoblastic leukemia with t(1;22)(p13;q13) is a rare malignancy occurring in infants and young children. The genes involved in t(1;22)(p13;q13) are unknown. In this study, dual-color fluorescence in situ hybridization (FISH) experiments with 15 probes were performed on the metaphase cells obtained from one patient to systematically narrow the region of the breakpoint on chromosome 22 and localize it to RP5-1042K10. A 22.3-kb FISH probe derived from RP5-1042K10 was used to further refine the locus of the breakpoint in this case. Southern blot analysis covering of genomic DNA from a second patient detected DNA rearrangement at a site close to the breakpoint observed with the 22.3-kb probe in the first case. A partially characterized gene, KIAA 1438, is in the vicinity of the breakpoints determined by FISH and Southern blot experiments, suggesting that this gene plays a role in this malignancy.