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By somatic cell fusion studies between noninvasive mouse T-lymphoma cells and invasive human activated normal T-cells we have previously shown that the genetic information responsible for the induction of invasive and metastatic potential in interspecies T-cell hybrids is located on human chromosome 7. Apparently, genes derived from normal activated T-cells are dominantly expressed in the hybrids and control the invasive and, as a consequence, metastatic potential of these T-lymphoma cells. To sublocalize the invasion-inducing locus on chromosome 7 we have generated hybrids that harbor only specific regions of human chromosome 7 with or without a small fragment of human chromosome 21. Analysis of these hybrids revealed that the invasion-inducing locus maps to 7p12→cen. The human DNA complement of the hybrids was confirmed by Southern blot analysis using a large panel of chromosome 7-specific DNA probes. Several of these genes could be further sublocalized. These included: ARAF2 to 7p12→cen, D7S21 to 7pter→p12, ACTB to 7p15→p12, EGFR to 7p12, MDH2 to 7cen→q22, and PDGFA to 7pter→p15.

The human synovial sarcoma-specific transloca-tion 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 SSX 1 and SSX2. This novel member of the family of KRAB containing SSX genes, which we designated SSX3, is 90 % homologous to SSX 1 and 95 % homologous to SSX2 at the cDNA level. Somatic ce11 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.