Mutations in the retinoblastoma gene (RBl) predispose to the formation of ocular tumors. Following the cloning of RBI in 1986 (1 ), polymorphic restriction enzyme sites within RBl were used to “track” the mutant gene within affected families using linkage analysis (2 , 3 ). This approach, however, was unsuitable for most retinoblastoma (Rb) patients because only 16% have a family history of Rb (4 ). In recent years, it has become possible to detect mutations in nucleic acids. The ability to detect mutations in RBl has important implications for Rb patients because it means that individuals predisposed to Rb but with no previous family history of the disease can now be screened for mutations.