Transcriptional repressor. May modulate the levels of histone H3K4Me3 by activating KDM5D histone demethylase. 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. Within the PRC1-like complex, regulates RNF2 ubiquitin ligase activity. Component of a PRC1-like complex. Interacts with BMI1/PCGF4, RING1 and RNF2. Interacts with KDM5D. Interacts with CBX4, CBX6, CBX7 and CBX8. Phosphorylated during mitosis.