The human SYT-SSX gene has two splicing isoforms (type N and 1), the latter of which contains an additional insertion of 93 bases. In the present study, we found increased transcriptional activity of the SYT-SSX type I protein in luciferase assay. When the SYT-SSX cDNAs were transfected to NIH3T3 cells, the type I transformant grew faster than the type N transformant. Furthermore, we evaluated the isoform ratio of the SYT or SYT-SSX transcripts in various tissues. Our results suggest that the SYT-SSX type I protein plays a critical role in the tumorigenesis of synovial sarcomas through increased transcriptional activity. (C) 2003 Elsevier Ireland Ltd. All rights reserved.