USER enzyme activity assay. A 34-mer oligonucleotide duplex (10 pmol) containing a single dU paired with a deoxyadenine ( Table 1 ) was incubated with a series of EndoVIII and UDG enzyme mixtures for 15 min at 37°C in a 10 μl of T4 DNA ligase reaction buffer (50 mM Tris-HCl pH 7.5, 10 mM MgCl 2 , 10 mM DTT, 1 mM ATP, 20 μg/ml BSA). The reactions were quenched by the addition of 10 μl of 95% formamide, 0.1% xylene cyanol, 0.1% bromophenol blue, 10 mM EDTA, pH 11, and the reaction products were analyzedon a 15% TBE-Urea denaturing gel. S, 34-nt oligonucleotide substrate. P1 and P2, 15 nt and 18 nt cleavage products, respectively. Two product bands of differing size result from the fact that the uracil is not in the centre of the substrate. Lane 1—no enzyme added. Lane 2—reaction contains 0.2 units of UDG. Lane 3—reaction contains 256 ng of EndoVIII. Lanes 4–10, reaction contains 0.2 units of UDG and the amount of EndoVIII shown above the respective lanes.