APEX1 (APE1, HAP1), a DNA apurinic/apyrimidinic (AP) site DNA lyase, cleaves the DNA strand sugar-phosphate backbone 5' to the AP site generated by DNA glycosylases or that forms by spontaneous loss of a base, producing a DNA strand with a 3'-terminal unsaturated sugar and a DNA strand with a terminal abasic 5'-deoxyribosephosphate (5'-dRP) as cleavage products (Wilson et al. 1995, Bennett et al. 1997, Masuda et al. 1998, Parikh et al. 1998).