Polycomb group (PcG) proteins represent transcriptional repressors that are present in single cell eukaryotes through multicellular organisms. These PcG complexes include Polycomb Repressive Complexes 1 and 2 (PRC1 and PRC2), as well as the more recently identified Pho-repressive Complex (PhoRC) and Polycomb Repressive Deubiquitinase (PR-DUB). The PRCs possess intrinsic histone modifying activities that contribute to their functions in transcriptional repression. PRC1 monoubiquitinates Lys119 in histone H2A (H2AK119ub1) and can compact chromatin by binding to nucleosomes, whereas PRC2 is a lysine methyltransferase (KMT) that trimethylates Lys27 in histone H3 (H3K27me3), a modification associated with PcG silencing.[1]