CpG site
![](/Images/godic/202501/03/Cpg_islands.svg0446.png")
![CpG,](/Images/godic/202501/03/CpG_vs_C-G_bp.svg0446.png")
![How methylation of CpG sites followed by spontaneous deamination leads to a lack of CpG sites in methylated DNA. As a result, residual CpG islands are created in areas where methylation is rare, and CpG sites stick (or where C to T mutation is highly detrimental).](/Images/godic/202501/03/Cytosine_becomes_thymine0446.png")
![An image showing a hypothetical evolutionary mechanism behind CpG island formation.](/Images/godic/202501/03/Cpg_island_evolution.svg0446.png")
The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG is shorthand for 5'—C—phosphate—G—3' , that is, cytosine and guanine separated by only one phosphate; phosphate links any two nucleosides together in DNA. The CpG notation is used to distinguish this single-stranded linear sequence from the CG base-pairing of cytosine and guanine for double-stranded sequences. The CpG notation is therefore to be interpreted as the cytosine being 5 prime to the guanine base. CpG should not be confused with GpC, the latter meaning that a guanine is followed by a cytosine in the 5' → 3' direction of a single-stranded sequence.