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. Required for proper control of cellular levels of GMNN expression (By similarity). Homodimer. Component of a PRC1-like complex (By similarity). Interacts with the SAM domain of SCMH1 via its SAM domain in vitro (PubMed:10653359). Interacts with RNF2 and CBX7 (PubMed:22226355). Interacts with PHC2 (PubMed:16024804). Interacts with BMI1 (PubMed:9009205). Highly expressed in testis with lower levels in most other tissues. Expressed in embryonic stem cells (PubMed:22226355). Expressed ubiquitously in 8.5 dpc embryos. At 10.5 dpc, strongly expressed in pharyngeal arches and weakly expressed in heart. By 14.5 dpc, expression is detected throughout the central nervous system. By retinoic acid.