CpG island

A CpG island is an area of the genome that has an elevated frequency of CpG sites, or instances of consecutive cytosine and guanine nucleotides. CpG islands occur frequently in promoters and at the start of transcription regions in many eukaryotes, and the identification of CpG islands is an important algorithmic problem when looking for coding regions of genes.

The "p" in "CpG island" represents the phosphodiester bond that joins the adjacent cytosine and guanine nucleotides in a strand of nucleic acid.