Polycomb group (PcG) protein. PcG proteins act by forming multiprotein complexes, which are required to maintain the transcriptionally repressive state of homeotic genes throughout development. PcG proteins are not required to initiate repression, but to maintain it during later stages of development. Component of the PcG multiprotein PRC1 complex, a complex that acts via chromatin remodeling and modification of histones; it mediates monoubiquitination of histone H2A 'Lys-118', rendering chromatin heritably changed in its expressibility. Needed to maintain expression patterns of the homeotic selector genes of the Antennapedia (Antp-C) and Bithorax (BX-C) complexes, and hence for the maintenance of segmental determination. Component of PRC1 complex, which contains many PcG proteins like Pc, ph, Scm, Psc, Sce and also chromatin-remodeling proteins such as histone deacetylases. This complex is distinct from the Esc/E(z) complex, at least composed of esc, E(z), Su(z)12, HDAC1/Rpd3 and Caf1-55. The 2 complexes however cooperate and interact together during the first 3 hours of development to establish PcG silencing.