Abstract A reciprocal chromosome translocation between 13q12 and 8p11 is the consistent cytogenetic abnormality seen in a nonspecific myeloproliferative disorder that is associated with T-cell leukemia/lymphoma and peripheral blood eosinophilia. Detailed molecular analyses of the translocation breakpoints associated with this rearrangement have not been reported to date. We have now generated somatic cell hybrids from a newly described patient with this specific structural rearrangement and analyzed the breakpoints on the derivative chromosomes. We have shown that the breakpoint on chromosome 13 lies within a 300- to 500-kb region defined by the KIAA177 gene and D13S1123 marker. In addition, we have identified a 1.2-Mb YAC, 959A4, that crosses the translocation breakpoint on the short arm of chromosome 8 in this patient. The location of this breakpoint in 8p11 is distinct from the t(8; 16) and t(8; 22) translocations associated with M4/M5 myeloid leukemias, and suggests that three distinct loci located within 8p11 are involved in the pathogenesis of myeloid neoplasias.