Fig. 4. Polymerase incorporating the first nucleotide into the repair gap is not sensitive to aphidicolin. Reaction mixtures (10 µl) containing 10 ng of substrate oligonucleotide duplex with normal (1–3) or reduced (4–6) AP sites were incubated with 10 µg of human cell extract supplemented with ddNTPs at 37°C in a reaction buffer described in Materials and methods. Reactions 2 and 5 contained 1% DMSO. Reactions 3 and 6 contained 1% DMSO and 100 μg/ml aphidicolin. Reaction products were analyzed by electrophoresis in 20% denaturing polyacrylamide gel.