More than 90% of synovial sarcomas (SSs) possess a non-random chromosomal translocation t(X;18)(p11;q11) and express SYT-SSX fusion gene products originating from the translocation. To test whether it is possible to detect the SYT-SSX mRNA on archival formalin-fixed paraffin-embedded SS tissue sections using PCR-based in situ amplification technique, we performed reverse transcription-polymerase chain reaction in situ hybridization (RT-PCR ISH) for the SYT-SSX fusion gene mRNA. In three types of SSs, monophasic fibrous, biphasic and poorly differentiated, NBT/BCIP signals corresponding to SYT-SSX mRNA were uniformly and predominantly positive in the sarcoma cell cytoplasm. Our results indicated that SS cells uniformly possessed the SYT-SSX fusion genes and expressed their transcripts. Furthermore, our results were thought to support monoclonal origin of epithelial and spindle cell components of biphasic SSs. Thus, specific chromosomal translocation t(X;18) is likely to be an early event in the development of SSs, and the expression of SYT-SSX fusion gene products is thought to be crucial for the tumorigenesis of SSs.

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