DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts to repress gene transcription. In mammals, DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprintingX-chromosome inactivation, repression of transposable elementsaging, and carcinogenesis

Abnormal methylation, often arising from lifestyle, environment, or other reasons can lead to cells being driven down the ‘cancer path’. This means methylation patterns provide one of the strongest signals to detect cancers, particularly the types and tissues of origin (TOO) of these cancers.