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CpG islands are regions of DNA near and in approximately 40% of promoters of mammalian genes. They are regions where a large concentration of phosphodiester-linked cytosine and guanine pairs exist. The "p" in CpG represents that they are phosphodiester-linked. Unlike CpG sites in the coding region of a gene, in most instances, the CpG sites in the CpG islands are unmethylated if genes are expressed. This observation led to the speculation that methylation of CpG sites in the promoter of a gene may inhibit the expression of a gene.
The usual formal definition of a CpG island is a region with at least 200 bp and with a GC percentage that is greater than 50% and with an observed/expected CpG ratio that is greater than 0.6.
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"CpG island"