The ability of the Endo IV assay to discriminate mismatches at different positions of a 14mer probe is illustrated in Figure 7 . Mismatches were introduced in the probe one at a time, from positions 1 to 8. The Endo IV assay shows excellent specificity for the different mismatches from base 1 to base 6 with large match/mismatch signal ratios. As shown in Figure 7 , the mismatch signal is often about the same as that of the NTC or slightly higher. The exceptions are the difficult 4-G/T-and 8-T/G-mismatches (24) where match/mismatch ratios of $16 and 7 were observed, respectively. These ratios are quite adequate to differentiate matched and mismatched sequences. It appears (data not shown) that the discrimination in positions 1 and 2 is largely determined by substrate requirements while discrimination in other positions is determined by thermodynamic considerations. Experiments with a large number of assays have also indicated that, with 10mer to 12mer probes, satisfactory discrimination is observed with the mismatch position in all except the last three bases at the 5 0 end.