Analysis of four YACs at the OATL1 locus was undertaken to determine the organization of locally repeated sequences within the OATL1 cluster. A restriction map of ICRFy900C0874, a 600-kb YAC, was constructed using a range of rare-cutting enzymes. Several markers were isolated from the OATL1 cluster, and the YAC map was used for their localization. Markers are shown to be reiterated within the OATL1 cluster, and fine-scale mapping has identified the accurate map position and organization of these repeated sequences within a 275-kb interval. The SSX1 gene involved in synovial sarcoma tumorigenesis and localized at the OATL1 cluster is also shown to be present in a minimum of five copies, all of which map to the aforementioned region. Several markers generated in this study have homologous counterparts at OATL2; they have been utilized to delimit the duplicated region at the OATL clusters. Although instability of one of the OATL1 YACs, ICRFy900F0501, has precluded a precise sizing of this interval, it has been possible to place an upper limit of 520 kb on this region of duplication.