Synovial sarcoma is characterized by the presence of a fusion protein involving SYT and SSX2. In this issue of Cancer Cell, Haldar et al. have genetically engineered a mouse model of this disease. They show that expression of the SYT-SSX2 fusion gene yields a highly penetrant and representative model of human synovial sarcoma, but only if expression occurs in a particular biologic context. The mouse model will be a valuable resource for studying tumor biology but is also a striking example of how important understanding of normal tissue and developmental biology is to our understanding of cancer.