Polycomb group (PcG) proteins maintain silencing at target loci in higher eukaryotes but recent evidence suggests that about half of these proteins are also required for maintenance of activation at homeotic loci.
The best-studied proteins required for the maintenance but not initiation of repression of target loci are encoded by genes of the Polycomb group (PcG). PcG mutants exhibit posterior transformations in embryos and adults caused by derepression of homeotic loci in flies , and in vertebrates . In addition, PcG genes regulate non-homeotic targets. In contrast, proteins of the trithorax group (trxG) are required for maintenance but not initiation of activation of homeotic loci . Many genes in the trxG were identified because they suppress phenotypes of PcG mutations, suggesting that the trxG proteins act antagonistically
to PcG proteins . Structural analysis of PcG and trxG response elements shows that the elements required for trxG-dependent activation and PcG-dependent repression are separable but within 30