DNA region of 500 base pairs that has a high CpG density and is usually unmethylated. CpG islands are found upstream of many mammalian genes; methylation leads to transcriptional silencing.

Repetitive CpG doublets creating a region of DNA greater than 200 bp in length with a G+C content of more than 0.5 and an observed/expected presence of CpG more than 0.6. Usually associated with transcription-initiation regions of (housekeeping) 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. On the other hand, many genes with restricted expression patterns have (methylated) CpG islands located downstream of transcription initiation does not block elongation of the transcript (see also Methylation paradox).

A DNA region characterized by methlyated cytosine residues in the sequence CpG. Methlated cytosines are correlated with reduced expression of genes.