The third stage of DNA demethylation is removal of the intermediate products of demethylation generated by a TET enzyme by base excision repair. As indicated above in Stage 2, after 5mC is first oxidized by a TET to form 5hmC, further oxidation of 5hmC by TET yields 5fC and oxidation of 5fC by TET yields 5caC. Both 5fC and 5caC are recognized by a DNA glycosylase, TDG, a base excision repair enzyme, as an abnormal base. As shown in the Figure in this section, TDG removes the abnormal base (e.g. 5fC) while leaving the sugar-phosphate backbone intact, creating an apurinic/apyrimidinic site, commonly referred to as an AP site. In this Figure, the 8-OHdG is left in the DNA, since it may have been present when OGG1 attracted TET1 to the CpG site with a methylated cytosine. After an AP site is formed, AP endonuclease creates a nick in the phosphodiester backbone of the AP site that was formed when the TDG DNA glycosylase removed the 5fC or 5caC. The human AP endonuclease incises DNA 5′ to the AP site by a hydrolytic mechanism, leaving a 3′-hydroxyl and a 5′-deoxyribose phosphate (5' dRP) residue. This is followed by either short patch or long patch repair. In short patch repair, 5′ dRP lyase trims the 5′ dRP end to form a phosphorylated 5′ end. This is followed by DNA polymerase β (pol β) adding a single cytosine to pair with the pre-existing guanine in the complementary strand and then DNA ligase to seal the cut strand. In long patch repair, DNA synthesis is thought to be mediated by polymerase δ and polymerase ε performing displacement synthesis to form a flap. Pol β can also perform long-patch displacement synthesis. Long-patch synthesis typically inserts 2–10 new nucleotides. Then flap endonuclease removes the flap, and this is followed by DNA ligase to seal the strand. At this point there has been a complete replacement of the 5-methylcytosine by cytosine (demethylation) in the DNA sequence.