Editing Cellular differentiation (section)

====Polycomb repressive complex (PRC2)====
In the realm of [[gene silencing]], [[PRC2|Polycomb repressive complex 2]], one of two classes of the [[Polycomb-group proteins|Polycomb group]] (PcG) family of proteins, catalyzes the di- and tri-methylation of histone H3 lysine 27 (H3K27me2/me3).<ref name= "Christophersen"/><ref name="Jiang">{{cite journal | vauthors = Zhu J, Adli M, Zou JY, Verstappen G, Coyne M, Zhang X, Durham T, Miri M, Deshpande V, De Jager PL, Bennett DA, Houmard JA, Muoio DM, Onder TT, Camahort R, Cowan CA, Meissner A, Epstein CB, Shoresh N, Bernstein BE | title = Genome-wide chromatin state transitions associated with developmental and environmental cues | journal = Cell | volume = 152 | issue = 3 | pages = 642–654 | date = January 2013 | pmid = 23333102 | pmc = 3563935 | doi = 10.1016/j.cell.2012.12.033 }}</ref><ref name = "Guenther">{{cite journal | vauthors = Guenther MG, Young RA | title = Transcription. Repressive transcription | journal = Science | volume = 329 | issue = 5988 | pages = 150–151 | date = July 2010 | pmid = 20616255 | pmc = 3006433 | doi = 10.1126/science.1193995 | bibcode = 2010Sci...329..150G }}</ref> By binding to the H3K27me2/3-tagged nucleosome, PRC1 (also a complex of PcG family proteins) catalyzes the mono-ubiquitinylation of histone H2A at lysine 119 (H2AK119Ub1), blocking [[RNA polymerase II]] activity and resulting in transcriptional suppression.<ref name= "Christophersen"/> PcG knockout ES cells do not differentiate efficiently into the three germ layers, and deletion of the PRC1 and PRC2 genes leads to increased expression of lineage-affiliated genes and unscheduled differentiation.<ref name= "Christophersen"/> Presumably, PcG complexes are responsible for transcriptionally repressing differentiation and development-promoting genes.