Key regulator of entry into cell division. Directly involved in heterochromatin formation by maintaining overall chromatin structure and, in particular, that of constitutive heterochromatin by stabilizing histone methylation. Controls histone H4 'Lys-20' trimethylation. Probably acts as a transcription repressor by recruiting chromatin-modifying enzymes to promoters (By similarity). Plays a dual role, regulating cell-cycle progression and transcriptional events leading to terminal differentiation. In the absence of a G1 phase, functions in late G2 controlling the expression of both S-phase and mitotic genes. Controls stalk/spore preference by suppressing the DIF response in cells destined for the spore pathway. DIF is a chlorinated hydroxyphenone made by cells of spore pathway that promotes stalk differentiation. In the growth phase, expression is correlated with several factors that lead to preference for the spore pathway. During multicellular development, expression increases 200-fold in differentiating spores. In shaking culture, leads to a higher density, a faster dying after reaching stationary density, and, after starvation, a lower spore viability. However, shows abnormal slow proliferation on bacterial lawns. Leads also to hypersensitivity to the stalk morphogen DIF. Belongs to the retinoblastoma protein (RB) family.