A CpG site on a genome is the term applied to cytosine and guanine appearing consecutively on the same strand of nucleic acid (the "p" stands for the phosphodiester bond joining the two nucleotides).
CpG sites are of practical interest because (especially in mammals), DNA methylation occurs very commonly on the cytosine belonging to the CG-pair, which serves as a key step in gene regulation by turning certain genes off.
Furthermore, when CpG sites occur frequently in a region of the genome, the region is called a CpG island. These regions appear frequently at the start of promoters in eukaryotes and generally can indicate the presence of a coding region.
