Rb is a tumor suppressor nuclear phosphoprotein capable of binding to DNA. It is phosphorylated on serine and threonine, but not on tyrosine residues. It forms a complex with SV40 large T antigen, adenovirus E1A, and human papilloma virus-16 E. Rb protein may act by regulating transcription and loss of its function leads to uncontrolled cell growth. Aberrations in the RB gene have been implicated in cancers of breast, colon, prostate, kidney, nasopharynx, and leukemia. Phosphorylation of Rb at S608 depends on CDK4.