Most naturally occurring mutants of the retinoblastoma (RB) protein contain large deletions or truncations. The small cell lung carcinoma cell line H209 contains a normal-sized but unphosphorylated RB protein (Hensel et al., Cancer Res., 50: 3067-3072, 1990), which fails to form a complex with SV40 T antigen, suggesting that the RB gene of H209 may contain a subtle mutation. To define this mutation, the RB complementary DNA and genomic DNA were sequenced, revealing a point mutation in exon 21 that changed a G to a T. This results in an amino acid substitution of a Phe for Cys706. The mutant RB complementary DNA was used as a template for in vitro transcription and translation to synthesize the mutated protein. The resulting protein failed to bind to SV40 T antigen, demonstrating that a single missense mutation of the RB gene led to the complete inactivation of the ability of the RB protein to bind T antigen.