Cytidine is a nucleoside formed by a cytosine attached to a ribose ring via a beta-N1-glycosidic bond. DNA is methylated on cytidines by DNA methylases (DNMTs)to generate 5-methylcytidine (5-mC), a potent epigenetics marker and regulator of gene expression. The reverse reaction (cytidine demethylation) starts with its oxidation to hydroxymethyl- (5-hmC), formyl- (5-fC), and carboxy- (5-caC) cytidine. Several enzymes, including the Tet family of proteins have been implicated in cytidine demethylation.