Rb (retinoblastoma 1) is a 105 kD nuclear protein containing RB-associated protein A and B domains. Rb acts as a transcriptional negative regulator of G1-phase cell cycle progression, and sequesters E2F and other transcription factors. Rb is unphosphorylated and inactive in G0, and becomes active upon hypophosphorylation by cyclinD-cdk4/6 in early G1. Rb is inactive with hyperphosphorylated by cyclinE-cdk2 late G1. Hypophosphorylated Rb form complexes with E2F, HDAC, cyclinD-cdk4/6, cyclinE-cdk2, PU 1, ATF 2, UBF, Elf 1, and c-Abl. The Poly6146 antibody recognizes the N-terminal region of human and mouse Rb and has been shown to be useful for Western blotting, immunoprecipitation, and immunofluorescence staining.