Chromosomal translocations are frequently associated with soft-tissue sarcomas. Fusion proteins generated by such translocations often play critical roles in tumorigenesis. Therefore, it is important to understand the function of the fusion protein to develop therapeutic interventions. The t(X;18)(p11.2;q11.2) translocation found in synovial sarcomas results in a fusion between the SYT gene on chromosome 18 and an SSX gene on the X chromosome. Although SYT-SSX fusion proteins appear to trigger synovial sarcoma development, little is known about the downstream targets of SYT-SSX. We found that the SYT-SSX fusion protein produces a dominant-negative function for SYT, which is a transcriptional coactivator. We then analyzed the gene expression profiles of SYT-SSX1-expressing HeLa cells using oligonucleotide microarrays and found that the SYT-SSX1 fusion protein directly down-regulated the expression of COM1, a regulator of cell proliferation. COM1 was found to be expressed at relatively low levels in synovial sarcoma tissues and cell lines. We then investigated the impact of conditional COM1 expression in the synovial sarcoma cell line. Increased COM1 expression resulted in induced apoptosis and in reduced cell growth and colony formation activity. Our results suggested that restoration of COM1 expression may be of therapeutic benefit in synovial sarcoma.Synovial sarcomas are aggressive tumors of adolescent and young adults that account for about 10% of all soft-tissue sarcomas. Synovial sarcomas are subdivided into biphasic and monophasic forms by their histomorphologic appearance. Biphasic synovial sarcomas contain epithelial cells arranged in glandular structures and spindle cells, whereas monophasic types are entirely composed of spindle cells. Cytogenetic analysis indicates that the chromosomal translocation t(X;18) (p11.2;q11.2) is present in the majority of these tumors, with molecular analysis of translocation breakpoints showing a disruption of the SYT (for "synovial sarcoma translocated") gene on chromosome 18q11.2 and juxtaposition to one of the SSX (for "synovial sarcoma X breakpoint") genes on Xp11.2 in a mutually exclusive fashion, with the result being a chimeric SYT-SSX protein (5, 9). The SSX gene family consists of nine contiguous members, SSX1 to SSX9, encoded on the X chromosome (12). Although the chromosomal arrangement of the SSX genes could in principle allow for heterogeneity at the breakpoint, fusions of SYT with SSX1, SSX2, and SSX4 have been reported in synovial sarcomas (6,8,9,23). The identity of the rearranged SSX gene has significant clinical impact (16).The normal SYT gene is ubiquitously expressed in a wide range of human tissues and cell lines (6). In contrast, SSX transcripts show a very restricted distribution in adult human tissues, and the expression is confined to the testis and at very low levels in the thyroid (6,11,26). SYT contains a transcriptional activating domain, rich in glycine, proline, glutamine, and tyrosine (QPGY domain). On the other hand, SSX possesses two tran...