The human synovial sarcoma-specific translocation t(X;18) results in the fusion of the SYT gene on chromosome 18 with either one of the Krüppel-associated box (KRAB) containing SSX1 or SSX2 genes on the X chromosome, depending on the exact location of the breakpoint within band Xp11.2. Screening of a testis cDNA library yielded several SSX-positive clones. Subsequent sequence analysis revealed that one third of these clones represent an SSX gene that differs from both SSX1 and SSX2. This novel member of the family of KRAB containing SSX genes, which we designated SSX3, is 90% homologous to SSX1 and 95% homologous to SSX2 at the cDNA level. Somatic cell hybrid analysis indicated that SSX3 maps within Xp11.2 --> p11.1, the region that also harbors the SSX1 and SSX2 genes. However, we conclude from our RT-PCR data and from results reported in the literature that SSX3 does not act as a fusion partner to SYT in any of the 44 independent synovial sarcomas thus far tested.