DNA cytosine methylation is a key epigenetic mark that is required for normal mammalian development. Iterative oxidation of 5-methylcytosine (5mC) by the TET family of DNA dioxygenases generates three oxidized nucleotides: 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC). Recent advances in genomic mapping techniques have suggested that these oxidized cytosines not only function in the process of active reversal of 5mC but also may possess unique regulatory functions in the mammalian genome.