Key regulator of entry into cell division (By similarity). 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 KMT5B and KMT5C, leading to epigenetic transcriptional repression. Controls histone H4 'Lys-20' trimethylation. Probably acts as a transcription repressor by recruiting chromatin-modifying enzymes to promoters (By similarity). Potent inhibitor of E2F-mediated trans-activation. May act as a tumor suppressor (By similarity). Component of the DREAM complex (also named LINC complex) at least composed of E2F4, E2F5, LIN9, LIN37, LIN52, LIN54, MYBL1, MYBL2, RBL1, RBL2, RBBP4, TFDP1 and TFDP2. The complex exists in quiescent cells where it represses cell cycle-dependent genes. It dissociates in S phase when LIN9, LIN37, LIN52 and LIN54 form a subcomplex that binds to MYBL2. Interacts with AATF. Interacts with KDM5A (By similarity). Interacts with KMT5B and KMT5C. Interacts with USP4 (By similarity). Interacts with RBBP9 (PubMed:9697699). Cell-cycle arrest properties are inactivated by phosphorylation on Thr-332, Ser-640, Ser-959 and Ser-970 by CDK4. Belongs to the retinoblastoma protein (RB) family.