Demethylation of 5-methylcytosine to generate 5-hydroxymethylcytosine (5hmC) very often initially involves oxidation of 5mC (see Figure in this section) by ten-eleven translocation methylcytosine dioxygenases (TET enzymes). The molecular steps of this initial demethylation are shown in detail in TET enzymes. In successive steps (see Figure) TET enzymes further hydroxylate 5hmC to generate 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC). Thymine-DNA glycosylase (TDG) recognizes the intermediate bases 5fC and 5caC and excises the glycosidic bond resulting in an apyrimidinic site (AP site). This is followed by base excision repair (stage 3). In an alternative oxidative deamination pathway, 5hmC can be oxidatively deaminated by APOBEC (AID/APOBEC) deaminases to form 5-hydroxymethyluracil (5hmU). Also, 5mC can be converted to thymine (Thy). 5hmU can be cleaved by TDG, MBD4, NEIL1 or SMUG1. AP sites and T:G mismatches are then repaired by base excision repair (BER) enzymes to yield cytosine (Cyt). The TET family of dioxygenases are employed in the most frequent type of demethylation reactions.