Editing Epigenetics (section)

===MicroRNAs===
[[MicroRNA]]s (miRNAs) are members of [[non-coding RNA]]s that range in size from 17 to 25 nucleotides. miRNAs regulate a large variety of biological functions in plants and animals.<ref name=Wang>{{cite journal | vauthors = Wang Z, Yao H, Lin S, Zhu X, Shen Z, Lu G, Poon WS, Xie D, Lin MC, Kung HF | title = Transcriptional and epigenetic regulation of human microRNAs | journal = Cancer Letters | volume = 331 | issue = 1 | pages = 1–10 | date = April 2013 | pmid = 23246373 | doi = 10.1016/j.canlet.2012.12.006 }}</ref> So far, in 2013, about 2000 miRNAs have been discovered in humans and these can be found online in a miRNA database.<ref>{{cite web| url = http://www.mirbase.org/cgi-bin/browse.pl| title = Browse miRBase by species<!-- Bot generated title -->}}</ref> Each miRNA expressed in a cell may target about 100 to 200 messenger RNAs(mRNAs) that it downregulates.<ref>{{cite journal | vauthors = Lim LP, Lau NC, Garrett-Engele P, Grimson A, Schelter JM, Castle J, Bartel DP, Linsley PS, Johnson JM | title = Microarray analysis shows that some microRNAs downregulate large numbers of target mRNAs | journal = Nature | volume = 433 | issue = 7027 | pages = 769–73 | date = February 2005 | pmid = 15685193 | doi = 10.1038/nature03315 | bibcode = 2005Natur.433..769L | s2cid = 4430576 }}</ref> Most of the downregulation of mRNAs occurs by causing the decay of the targeted mRNA, while some downregulation occurs at the level of translation into protein.<ref>{{cite journal | vauthors = Lee D, Shin C | title = MicroRNA-target interactions: new insights from genome-wide approaches | journal = Annals of the New York Academy of Sciences | volume = 1271 | issue = 1 | pages = 118–28 | date = October 2012 | pmid = 23050973 | pmc = 3499661 | doi = 10.1111/j.1749-6632.2012.06745.x | bibcode = 2012NYASA1271..118L }}</ref>

It appears that about 60% of human protein coding genes are regulated by miRNAs.<ref>{{cite journal | vauthors = Friedman RC, Farh KK, Burge CB, Bartel DP | title = Most mammalian mRNAs are conserved targets of microRNAs | journal = Genome Research | volume = 19 | issue = 1 | pages = 92–105 | date = January 2009 | pmid = 18955434 | pmc = 2612969 | doi = 10.1101/gr.082701.108 }}</ref> Many miRNAs are epigenetically regulated. About 50% of miRNA genes are associated with [[CpG island]]s,<ref name=Wang /> that may be repressed by epigenetic methylation. Transcription from methylated CpG islands is strongly and heritably repressed.<ref>{{cite journal | vauthors = Goll MG, Bestor TH | title = Eukaryotic cytosine methyltransferases | journal = Annual Review of Biochemistry | volume = 74 | pages = 481–514 | year = 2005 | pmid = 15952895 | doi = 10.1146/annurev.biochem.74.010904.153721 | s2cid = 32123961 }}</ref> Other miRNAs are epigenetically regulated by either histone modifications or by combined DNA methylation and histone modification.<ref name=Wang />