DNA methylation is a process by which methyl groups are added to DNA. Methylation modifies the function of the DNA. When located in a gene promoter, DNA methylation typically acts to repress gene transcription.
DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, repression of repetitive elements, aging and carcinogenesis.
Attribution: Christoph Bock (Max Planck Institute for Informatics) (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
This means that every cell type has a unique DNA methylation fingerprint that changes during normal biological processes and many diseases, in particular cancer. Two of DNA’s four nucleotides, cytosine and adenine, can be methylated.