Post updated 2014 January 2nd DNA methylation is a biochemical process and epigenetic modification, whereby a methyl group is added to the cytosine nucleotide (and also adenine) to form 5-methylcytosine. DNA methylation at the 5' position of cytosine has the specific effect of reducing gene transcription and typically occurs in CpG sites, which are regions of DNA where a cytosine occurs next to a