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. In the PRC1 complex, it is required to stimulate the E3 ubiquitin-protein ligase activity of rnf2 (By similarity). Component of a PRC1-like complex (By similarity). Interacts with cbx4. Maternally derived transcript is detected at high levels in blastula. Detected at intermediate levels in the head region, the rhombencephalon, otic vesicle, mesencephalon, optic vesicle and in the anterior parts of the spinal cord in late neurula stages.