C-phosphodiester-G

CpG sites are regions of DNA where a cytosine nucleotide occurs next to a guanine nucleotide in the linear sequence of bases along its length. "CpG" is shorthand for "—C—phosphate—G—", that is, cytosine and guanine separated by a phosphate, which links the two nucleosides together in DNA. The "CpG" notation is used to distinguish this linear sequence from the CG base-pairing of cytosine and guanine.

There are regions of the DNA that have a higher concentration of CpG sites, known as CpG islands. Many genes in mammalian genomes have CpG islands associated with the start of the gene.[4] Because of this, the presence of a CpG island is used to help in the prediction and annotation of genes. These increased concentrations of CpGs might be associated with the decreased methylation of cytosines often observed in CpG islands — this could result in a reduced vulnerability to transition mutations and, as a consequence, a higher equilibrium density of CpGs surviving.

Methylation of CpG sites within the promoters of genes can lead to their silencing, a feature found in a number of human cancers (for example the silencing of tumor suppressor genes).

http://en.wikipedia.org/wiki/CpG_site