MOLECULAR BIOLOGY: CHROMATIN DNA PACKAGING AND GENE SILENCING:
"One basic premise of chromatin regulation is that genes are silenced through compaction of chromatin, which reduces the accessibility of DNA. In contrast, gene expression may require the "opening up" of chromatin. The Polycomb group (PcG) of gene repressors and the trithorax group (trxG) of gene activators are two antagonistic classes of proteins that may act through modulation of chromatin structure. Together, these factors maintain the gene expression patterns of key developmental regulators and hence are crucial players in cellular differentiation, stem cell renewal, and cancer."