AbstractThe retinoblastoma susceptibility (RB) gene is unique among other cloned cancer genes because its causal role in a human cancer, retinoblastoma, was established by classical genetic methods before its isolation. Earlier hypotheses and experimental data suggested that inactivation of a gene in chromosome band 13q14 resulted in retinoblastoma formation. A gene in this region was identified as the RB gene on the basis of mutations found specifically in retinoblastoma tumors; however, its proposed biological activity in suppressing neoplasia has yet to be demonstrated. The RB gene product was identified as a nuclear phosphoprotein of 110 kD associated with DNA binding activity, suggesting that the RB protein may regulate other genes. Probes for the RB gene and gene product will be useful for genetic diagnosis of retinoblastoma susceptibility in affected families; for direct detection of mutant RB alleles; and, potentially, for genetic diagnosis of susceptibility to osteosarcoma and other tumors tentatively linked to RB‐gene dysfunction. Continued study of the RB gene should yield further insight into mechanisms of oncogenesis, development, and gene regulation.