Fig. (1) DNA methylation and DNA hydroxymethylation. DNMTs catalyses the transfer of a methyl group from SAM to the fifth carbon of C creating 5mC. The methyl group of 5mC can be modified by the addition of a hydroxyl group mediated by TETs to generate 5-hydroxymethyl-cytosine (5hmC).  DNMTs: DNA methyltransferases; SAM: S-adenosyl methionine; SAH: S-adenosyl homocysteine; C: cytosine; 5mC: 5-methylcytosine; 5hmC: 5-hydroxymethyl-cytosine; TETs: ten-eleven translocation cytosine dioxygenases