E3 ubiquitin-protein ligase that mediates monoubiquitination of 'Lys-118' of histone H2A, thereby playing a central role in histone code and gene regulation. H2A 'Lys-118' ubiquitination gives a specific tag for epigenetic transcriptional repression. 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. PcG complexes act via modification of histones, such as methylation, deacetylation, ubiquitination rendering chromatin heritably changed in its expressibility. May play a role in meiotic sister chromatid cohesion. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Interacts with ORD. 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 two complexes however cooperate and interact together during the first 3 hours of development to establish PcG silencing. Colocalizes with ubiquitinated histone H2A. Colocalizes with Pc, Pcl, Psc, ph at many sites on polytene chromosomes. Colocalizes with ORD on the chromatin of primary spermatocytes during G2 of meiosis. Ubiquitously expressed in syncytial blastoderm embryos. Ubiquitously expressed until stage 11. Then, it is only expressed in the neuroectoderm. Later in embryonic development, it is only expressed in the CNS. In larvae, it is expressed in all imaginal disks. Expressed in the male and female gonads. Expressed both maternally and zygotically.