Tumor suppressor that is a key regulator of the G1/S transition of the cell cycle. 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. Recruits and targets histone methyltransferases SUV39H1, KMT5B and KMT5C, leading to epigenetic transcriptional repression. Controls histone H4 'Lys-20' trimethylation. 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 (PubMed:19081374). 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). The hypophosphorylated form interacts with and sequesters the E2F1 transcription factor. Interacts with heterodimeric E2F/DP transcription factor complexes containing TFDP1 and either E2F1, E2F3, E2F4 or E2F5, or TFDP2 and E2F4. 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 and TMPO-alpha. May interact with NDC80. Interacts with GRIP1 and UBR4. Interacts with ARID4A and KDM5B. Interacts with E4F1, LIMD1 and USP4. Interacts with PRMT2. Interacts (when methylated at Lys-852) with L3MBTL. Interacts with CHEK2; phosphorylates RB1 (By similarity). Interacts with SMARCA4/BRG1 and HDAC1. Binds to CDK1 and CDK2. P-TEFB complex interacts with RB1; promotes phosphorylation of RB1 (By similarity). Interacts with RBBP9; the interaction disrupts RB1 binding to E2F1 (PubMed:9697699). Interacts with KAT2B/PCAF and EP300/P300 (By similarity). Interacts with PAX5 (By similarity). Phosphorylated by CDK6 and CDK4, and subsequently by CDK2 at Ser-560 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. CDK3/cyclin-C-mediated phosphorylation at Ser-799 and Ser-803 is required for G0-G1 transition (By similarity). Dephosphorylated at Ser-787 by calcineruin upon calcium stimulation. N-terminus is methylated by METTL11A/NTM1. Monomethylation at Lys-802 by SMYD2 enhances phosphorylation at Ser-799 and Ser-803, and promotes cell cycle progression. Monomethylation at Lys-852 by SMYD2 promotes interaction with L3MBTL1 (By similarity). Acetylated during keratinocyte differentiation. Acetylation at Ser-799 and Ser-803 regulates subcellular localization. Can be deacetylated by SIRT1. Belongs to the retinoblastoma protein (RB) family.