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{{distinguish|CpG oligodeoxynucleotide}} {{short description|Region of often-methylated DNA with a cytosine followed by a guanine}} [[File:CpG vs C-G bp.svg|300px|thumb|a CpG site, ''i.e.'', the " 5'βCβphosphateβGβ3' " sequence of nucleotides, is indicated on one DNA strand (in yellow). On the reverse DNA strand (in blue), the complementary 5'βCpGβ3' site is shown. A C-G base-pairing between the two DNA strands is also indicated (right)]] The '''CpG sites''' or '''CG sites''' are regions of [[DNA]] where a [[cytosine]] [[nucleotide]] is followed by a [[guanine]] nucleotide in the linear [[DNA sequence|sequence]] of [[Base pair|base]]s along its [[Directionality (molecular biology)|5' β 3' direction]]. CpG sites occur with high frequency in genomic regions called [[CpG site#CpG islands|CpG islands]]. Cytosines in CpG dinucleotides can be [[DNA methylation|methylated]] to form [[5-methylcytosine]]s. [[Enzyme]]s that add a [[methyl group]] are called [[DNA methyltransferase]]s. In mammals, 70% to 80% of CpG cytosines are methylated.<ref name="Jabbari2004">{{cite journal |vauthors=Jabbari K, Bernardi G |title=Cytosine methylation and CpG, TpG (CpA) and TpA frequencies |journal=Gene |volume=333 |pages=143β9 |date=May 2004 |pmid=15177689 |doi=10.1016/j.gene.2004.02.043 }}</ref> Methylating the cytosine within a gene can change its expression, a mechanism that is part of a larger field of science studying gene regulation that is called [[epigenetics]]. Methylated cytosines often mutate to [[Thymine|thymines]]. In humans, about 70% of [[Promoter (genetics)|promoters]] located near the [[Transcription (genetics)|transcription start site]] of a gene (proximal promoters) contain a CpG island.<ref name="pmid16432200">{{cite journal |vauthors=Saxonov S, Berg P, Brutlag DL |title=A genome-wide analysis of CpG dinucleotides in the human genome distinguishes two distinct classes of promoters |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=103 |issue=5 |pages=1412β7 |year=2006 |pmid=16432200 |pmc=1345710 |doi=10.1073/pnas.0510310103 |bibcode=2006PNAS..103.1412S |doi-access=free }}</ref><ref name="pmid21576262">{{cite journal |vauthors=Deaton AM, [[Adrian Bird|Bird A]] |title=CpG islands and the regulation of transcription |journal=Genes Dev. |volume=25 |issue=10 |pages=1010β22 |year=2011 |pmid=21576262 |pmc=3093116 |doi=10.1101/gad.2037511 }}</ref>
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