Transcriptional repressor (PubMed:12167161). May modulate the levels of histone H3K4Me3 by activating KDM5D histone demethylase (PubMed:17320162). Component of a Polycomb group (PcG) multiprotein PRC1-like complex, a complex class required to maintain the transcriptionally repressive state of many genes, including Hox genes, throughout development. PcG PRC1 complex acts via chromatin remodeling and modification of histones; it mediates monoubiquitination of histone H2A 'Lys-119', rendering chromatin heritably changed in its expressibility (PubMed:12167161). Within the PRC1-like complex, regulates RNF2 ubiquitin ligase activity (PubMed:26151332). Component of a PRC1-like complex (PubMed:19636380, PubMed:26151332). Interacts with BMI1/PCGF4, RING1 and RNF2 (PubMed:12167161). Interacts with KDM5D (PubMed:17320162). Interacts with CBX4, CBX6, CBX7 and CBX8 (PubMed:21282530). Ubiquitous. Phosphorylated during mitosis. Phosphorylated on Ser-30 by CDK7 in vitro.