CpG island

CpG islands are genomic regions that contain a high frequency of CG dinucleotides. In mammalian genomes, CpG islands are typically 300-3,000 base pairs in length. They are in and near approximately 40% of promoters of  mammalian genes (about 70% in human promoters). The "p" in CpG notation refers to the phosphodiester bond between the cytosine and the guanine.
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Repetitive CpG doublets occurring in a sea of DNA sequences low in CpG residues. Usually associated with transcription-initiation regions of genes transcribed at low rates that do not contain a TATA box. The CpG-rich stretch of 20-50 nucleotides occurs within the first 100-200 bases upstream of the start site region (where promoter-proximal elements reside). A trans-acting transcription factor called SP1 recognizes the CpG islands (see also Htf islands). In vertebrates, many of the nontranscribed genes (and the genes on the inactivated X chromosome) have a 5-methyl group on the C residue in CpG dinucleotides in transcription-control regions.