It is now well recognized that chromosomal translocation followed by overexpression of a chimeric gene product plays a critical role in tumorigenicity in various malignant tumors, especially those of leukemia, malignant lymphoma, and soft-tissue tumors. In these malignant tumors, specific chimeric gene products are directly related to tumorigenicity. Therefore, if chimeric gene products could be observed in situ, it would be advantageous not only for the correct diagnosis of each tumor but also to improve our understanding of the basis of tumorigenicity. Accordingly, it would seem that reverse transcriptase (RT) in situ polymerase chain reaction (PCR) is a powerful and useful approach for the study of chimeric gene products in situ. Here, we introduce the application of RT in situ PCR to detect a hybrid, SYT-SSX messenger RNA in synovial sarcoma. We expect that the principle of this protocol also may be applied to detect other chimeric gene products.