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. Interacts with and sequesters the E2F1 transcription factor. Interacts with SUV39H1, KMT5B and KMT5C (By similarity). (Microbial infection) Interacts with, and is inhibited by fowl adenovirus 1 protein GAM-1. Phosphorylated in G1, thereby releasing E2F1 which is then able to activate cell growth. Dephosphorylated at the late M phase. Phosphorylation of 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 (By similarity). Belongs to the retinoblastoma protein (RB) family.