الفهرس 
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Abstract
Polycomb group (PcG) proteins are conserved transcriptional repressors that cooperate to stably maintain the silenced state of several developmental regulator genes, in cells in which these genes should not be expressed.
They were first discovered in 1947 by Pamela Lewis in experiments performed on fruit flies. PcG proteins are conserved in higher eukaryotes and act by forming multiprotein complexes. Five PcG complexes have been identified. These are termed polycomb repressive complexes 1 (PRC1), polycomb repressive complexes 2 (PRC2), pleiohomeotic-repressive complex (PhoRC), dRing-associated factors (dRAF) complex and polycomb repressive deubiquitinase (PR-DUB) complex. PRC1 and PRC2 were the first to be identified.
PcG proteins can be organized as microscopically visible foci in the nucleus called PcG bodies due to existence of numerous multi- gene clusters and the insulator-like elements which causes the interactions between remote PcG targets.
Many proteins have been identified to interact with PcG complexes called polycomb group associated factors. These factors are involved in many other aspects of gene regulation. They include the transcription factor E2F6, C-terminal binding protein and some histone deacetylases.
The polycomb group expression varies greatly among tissues and even among specific cell types within a particular tissue. Furthermore, each polycomb group gene is expressed in a different number of tissues.
Tumor cells maintain the expression pattern of their normal counterparts. So, RING1which is normally expressed in nearly all tissues, it is the most ubiquitously detected PcG protein in human tumors. In addition, HPH1which is normally expressed in only a few tisssues, it is undetectable in most tumors.
Polycomb group complexes have specific catalytic functions believed to be important for their ability to repress transcription and their biological activity e.g.: tri-methylation of histone H3 at lysine 27 by PRC2, the ubiquitylation and sumoylation activity of PRC1.
Conclusion: PcG proteins function as epigenetic switches on chromatin that delineate both repressive and derepressive gene signals, thereby maintaining a particular cell identity or allowing differentiation. PcG complexes regulate multiple cellular processes. Such processes involve cell cycle checkpoints and progression, DNA damage repair, differentiation, apoptosis, senescence, X-chromosome inactivation and genomic imprinting. Deregulation of PcG proteins activity is central to cancer development and cancer stem cell biology. PcGs are considered as novel prognostic markers to predict poor therapeutic outcome for patients with various types of cancer. Pharmacological targeting of PcG proteins is emerging as an attractive strategy.