Key regulator of entry into cell division that acts as a tumor suppressor. Promotes G0-G1 transition when phosphorylated by CDK3/cyclin-C. Acts as a transcription repressor of E2F1 target genes. The underphosphorylated; active form of RB1 interacts with E2F1 and represses its transcription activity; leading to cell cycle arrest. 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; SUV420H1 and SUV420H2; 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. 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). In case of viral infections; interactions with SV40 large T antigen; HPV E7 protein or adenovirus E1A protein induce the disassembly of RB1-E2F1 complex thereby disrupting RB1's activity. {ECO:0000250; ECO:0000269|PubMed:15084261}.