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. Expressed in ovary, testis, stomach, liver, thymus and kidney (at protein level). Expressed in embryo at 10.5 dpc. Phosphorylated during mitosis.