Reconstitution assay using purified BER proteins. ( A ) Wild-type (WT) and variant APE1 proteins were incubated with UDG and POLβ with 32 P-labeled 34U DNA substrate (1 pmol), and the reactions were resolved on a urea-polyacrylamide denaturing sequencing gel. The non-incised substrate (S), AP site incision product (P1), and gap-filling extension product (P2) were visualized and quantified using standard phosphorimaging analysis. NE = no enzyme. ( B ) Relative AP site cleavage efficiency. Shown are the averages and standard deviations of 4 independent reactions. ( C ) Relative gap-filling activity. Shown are the average and standard deviation of 4 independent assays.