In a previous study we localized the synovial sarcoma-associated t(X;18)(p11;q11) breakpoint within the ornithine aminotransferase-like 1 (OATL1) cluster on the X chromosome. This localization was delineated from both somatic cell hybrid and fluorescence in situ hybridization (FISH) analysis of patient material, using OAT-specific cDNA and YAC probes. Simultaneously, Knight et al. (1992, Mol. Hum. Genet, in press) mapped this same breakpoint in their patient material adjacent to the more proximal OATL2 region on the X chromosome. Here we report the analysis of two additional tumors and demonstrate that again in these cases the chromosomal break occurs within the OATL1 cluster. In order to further specify the breakpoint, we subcloned the OATL1 YAC (no. 2) into cosmids. At least one of these cosmids (0.38) hybridizes to sequences that bracket the translocation breakpoint, as demonstrated by both Southern blot and FISH analysis. These observations confirm and substantiate our previous findings. In addition, cosmid 0.38 should be a valuable instrument for the ultimate isolation and identification of the gene(s) involved in the development of synovial sarcoma.