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 (5mC). In somatic cells, 5mC 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 5mC is also observed in non-CpG contexts. The biological importance of 5mC as amajor epigenetic modification in phenotype and gene expression has been recognized widely.

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

The TET family of 5mC hydroxylases includes TET1, TET2 and TET3. These TET proteins may promote DNA demethylation by binding to CpG-rich regions to prevent unwanted DNA methyltransferase activity, and by converting 5mC to 5hmC and further to 5-carboxylcytosine (5-caC) through hydroxylase activity. It was shown that genomic 5hmC level correlates to TET hydroxylase activity. In addition, TET1 was shown to have dual functions in transcription activation and repression by binding different target genes in ES cells. TET1 is also a fusion partner of the MLL gene in acute myeloid leukemia and is considered an oncoprotein. TET2 is found to be frequently mutated in leukemia and considered to act as tumor suppressor. TET3 has been demonstrated to play a unique role for DNA methylation reprogramming processes in the mammalian zygote. Thus, activating tumor suppressor TET enzymes such as TET2 or inhibiting oncoprotein TET enzymes such as TET1 would be important in benefiting cancer diagnostics and developing new target-based cancer therapeutics.