English: Two strands of DNA are represented by parallel horizontal lines. The first downward arrow shows thymine DNA glycosylase (TDG) removing 5-formylcytosine (5fC) from the DNA backbone, leaving an apyrimidinic site. Then AP endonuclease cleaves the 5′ deoxyribose-phosphate in the DNA backbone of a single strand, leaving a 3′ hydroxy end and a 5′ deoxyribose phosphate end (second downward arrow). 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 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 displacing synthesis to form a flap. Pol β can also perform long-patch displacing 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.