PcG proteins regulate chromatin structure through post-translational modifications, particularly histone tail methylation. PcG proteins and their functional antagonists, TrxG proteins, produce a bivalent state that is responsive to signaling events. Ancillary DNA-binding proteins, such as YY1 in the Pho repressive complex, aid recruitment of PRC1 and PRC2 to target genes. They interact with DNA and chromatin modifiers, including DNA methyl transferases, histone deacetylases, histone acetyltransferases, and the Jarid pathway proteins. H2A, histone 2A; H3, histone 3; Me, methyl group; PRC, polycomb repressive complex; Ubq, ubiquitin; PhoRC, Pho repressive complex. (From Reynolds et al., Transcriptional response of polycomb group genes to status epilepticus in mice is modified by prior exposure to epileptic preconditioning, Frontiers in Neurology, 6:46, 2015)