DNA methylation occurs by the covalent addition of a methyl group at the 5-carbon of the cytosine ring by DNA methyltransferases, resulting in 5-methylcytosine (5-mC). In somatic cells, 5-mC is found almost exclusively in the context of paired symmetrical methylation of the dinucleotide CpG, whereas in embryonic stem (ES) cells, a substantial amount of 5-mC is also observed in non-CpG contexts. The biological importance of 5-mC as a major epigenetic modification in phenotype and gene expression has been recognized widely.

5-hydroxymethylcytosine (5-hmC), as a sixth DNA base with functions in transcription regulation, has been detected to be abundant in human and mouse brains and embryonic stem (ES) cells. In mammals, it can be generated by the oxidation of 5-mC, a reaction mediated by the ten-eleven translocation (TET) family of 5-mC-hydroxylases.

5-hmC has been demonstrated to be tissue specific, ranging from undetectable levels in cultured cell lines to 0.6% in human brain tissues, and can be as high as 8% of total DNA in some other species. The biological significance of 5-hmC as an important epigenetic modification in phenotype and gene expression has been recently recognized. For example, global decrease in 5-hmC content (DNA hypohydroxymethylation) has been exhibited in nearly all cancers and has been proposed as a molecular marker and therapeutic target in cancer as well.