Tumor suppressor that is a key regulator of the G1/S transition of the cell cycle (PubMed:8336704). The hypophosphorylated form binds transcription regulators of the E2F family, preventing transcription of E2F-responsive genes. Both physically blocks E2Fs transactivating domain and recruits chromatin-modifying enzymes that actively repress transcription. Cyclin and CDK-dependent phosphorylation of RB1 induces its dissociation from E2Fs, thereby activating transcription of E2F responsive genes and triggering entry into S phase. RB1 also promotes the G0-G1 transition upon phosphorylation and activation by CDK3/cyclin-C. Directly involved in heterochromatin formation by maintaining overall chromatin structure and, in particular, that of constitutive heterochromatin by stabilizing histone methylation (PubMed:15750587). Recruits and targets histone methyltransferases SUV39H1, KMT5B and KMT5C, leading to epigenetic transcriptional repression. Controls histone H4 'Lys-20' trimethylation (PubMed:16612004). Inhibits the intrinsic kinase activity of TAF1. Mediates transcriptional repression by SMARCA4/BRG1 by recruiting a histone deacetylase (HDAC) complex to the c-FOS promoter. In resting neurons, transcription of the c-FOS promoter is inhibited by BRG1-dependent recruitment of a phospho-RB1-HDAC1 repressor complex. Upon calcium influx, RB1 is dephosphorylated by calcineurin, which leads to release of the repressor complex (By similarity) (PubMed:15750587, PubMed:16612004, PubMed:8336704). The hypophosphorylated form interacts with and sequesters the E2F1 transcription factor. Interacts with heterodimeric E2F/DP transcription factor complexes containing TFDP1 and either E2F1/E2F, E2F3, E2F4 or E2F5, or TFDP2 and E2F4 (PubMed:8336704, PubMed:20940255). The unphosphorylated form interacts with EID1, ARID3B, KDM5A, SUV39H1, MJD2A/JHDM3A and THOC1. Interacts with the N-terminal domain of TAF1. Interacts with SNW1, ATAD5, AATF, DNMT1, LIN9, LMNA, KMT5B, KMT5C, PELP1, UHRF2, TMPO-alpha and USP4. May interact with NDC80. Interacts with GRIP1 and UBR4. Interacts with ARID4A and KDM5B. Interacts with E4F1 and LIMD1. Interacts with SMARCA4/BRG1 and HDAC1. Interacts with USP4. Interacts (when methylated at Lys-853) with L3MBTL1. Binds to CDK1 and CDK2. Interacts with CHEK2; phosphorylates RB1 (By similarity). Interacts with PRMT2. Interacts with CEBPA. P-TEFB complex interacts with RB1; promotes phosphorylation of RB1 (By similarity). Interacts with RBBP9; the interaction disrupts RB1 binding to E2F1 (By similarity). Interacts with KAT2B/PCAF and EP300/P300 (PubMed:20940255). Interacts with PAX5 (By similarity). (Microbial infection) Interacts with adenovirus E1a protein. (Microbial infection) Interacts with SV40 large T antigen. During keratinocyte differentiation, acetylation by KAT2B/PCAF is required for nuclear localization. Expressed in the cell nuclei of renal tubules, hepatocytes and skeletal muscles. Expressed in skin (at protein level) (PubMed:20940255). Phosphorylated (PubMed:8336704). Phosphorylated by CDK6 and CDK4, and subsequently by CDK2 at Ser-561 in G1, thereby releasing E2F1 which is then able to activate cell growth. Dephosphorylated at the late M phase. Phosphorylation of threonine residues in domain C promotes interaction between the C-terminal domain C and the Pocket domain, and thereby inhibits interactions with heterodimeric E2F/DP transcription factor complexes. Dephosphorylated at Ser-788 by calcineruin upon calcium stimulation. CDK3/cyclin-C-mediated phosphorylation at Ser-800 and Ser-804 is required for G0-G1 transition (By similarity). Phosphorylated by CDK1 and CDK2 upon TGFB1-mediated apoptosis (By similarity). Monomethylation at Lys-803 by SMYD2 enhances phosphorylation at Ser-800 and Ser-804, and promotes cell cycle progression. Monomethylation at Lys-853 by SMYD2 promotes interaction with L3MBTL1 (By similarity). N-terminus is methylated by METTL11A/NTM1. Acetylated in the skin (PubMed:20940255). Acetylation at Lys-866 and Lys-867 regulates subcellular localization during keratinocytes differentiation (By similarity). Belongs to the retinoblastoma protein (RB) family. Retinoblastoma protein entry