Editing Heterochromatin (section)

==Facultative heterochromatin==
[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyotype|karyogram]] of a [[human]], showing an overview of the [[human genome]] using [[G banding]], which is a method that includes [[Giemsa stain]]ing, wherein the lighter staining regions are generally more euchromatic, whereas darker regions generally are more heterochromatic.{{further|Karyotype}}]]
The regions of DNA packaged in facultative heterochromatin will not be consistent between the cell types within a species, and thus a sequence in one cell that is packaged in facultative heterochromatin (and the genes within are poorly expressed) may be packaged in euchromatin in another cell (and the genes within are no longer silenced). However, the formation of facultative heterochromatin is regulated, and is often associated with [[morphogenesis]] or [[Cellular differentiation|differentiation]]. An example of facultative heterochromatin is [[X-inactivation|X chromosome inactivation]] in female mammals: one [[X chromosome]] is packaged as facultative heterochromatin and silenced, while the other X chromosome is packaged as euchromatin and expressed.

Among the molecular components that appear to regulate the spreading of heterochromatin are the [[Polycomb-group proteins]] and non-coding genes such as [[Xist]]. The mechanism for such spreading is still a matter of controversy.<ref>{{cite journal | vauthors = Talbert PB, Henikoff S | title = Spreading of silent chromatin: inaction at a distance | journal = Nature Reviews. Genetics | volume = 7 | issue = 10 | pages = 793–803 | date = October 2006 | pmid = 16983375 | doi = 10.1038/nrg1920 | s2cid = 1671107 }}
</ref>  The polycomb repressive complexes [[polycomb repressive complex 1|PRC1]] and [[PRC2]] regulate [[chromatin]] compaction and gene expression and have a fundamental role in developmental processes.  PRC-mediated [[epigenetics|epigenetic]] aberrations are linked to [[genome instability]] and malignancy and play a role in the [[DNA damage (naturally occurring)|DNA damage]] response, [[DNA repair]] and in the fidelity of [[DNA replication|replication]].<ref name="pmid28758948">{{cite journal | vauthors = Veneti Z, Gkouskou KK, Eliopoulos AG | title = Polycomb Repressor Complex 2 in Genomic Instability and Cancer | journal = International Journal of Molecular Sciences | volume = 18 | issue = 8 | pages = 1657 | date = July 2017 | pmid = 28758948 | pmc = 5578047 | doi = 10.3390/ijms18081657 | doi-access = free }}</ref>