CpG island
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CpG islands are regions of DNA near and in the promoter of a eukaryotic gene where a large concentration of CpG sites exist. Unlike CpG sites in the coding region of a gene, in most instances the CpG sites in the CpG islands are un-methylated if genes are expressed. This raises the possibility that methylation of CpG may inhibit the expression of a gene by altering the promoter.
External Link:
M. D. Anderson Cancer Center (http://www.mdanderson.org/departments/methylation/dIndex.cfm?pn=F68B486D-E00C-4398-9A7D2FE1F78EC9B9)