Methods: Double-blind studies were conducted in genotype 1 HCVinfected, treatment-naive subjects. In trial 003, subjects received placebo (n =8) or IDX184 (n =33) at doses of 25 to 100mg/day for 3 days. In trial 004, subjects received placebo (n =16) or IDX184 (n =63) at doses of 50 to 200mg/day for 14 days. Samples obtained at baseline, 1 day and 2 weeks after final dosing were analyzed by NS5B population sequencing. Two 003 subjects were further evaluated by clonal sequencing. Results: No S282T mutation was detected by population sequencing in any baseline samples in the two studies. No S282T emerged following either 3 or 14 days of treatment with IDX184 or placebo. Clonal analysis of samples from the 2 003 subjects who had modest rebound (0.5 to 0.7 log10 IU/mL) during IDX184 treatment did not detect the S282T mutation in any of approximately 200 baseline or post-treatment samples. Evaluation of all NS5B codons across all samples from both trials revealed a number of treatment-emergent changes in both placebo and IDX184 subjects. These appeared to be random and generally occurred at sites that were polymorphic or showed mixed residues at baseline. No net change at any residue was attributed to treatment with IDX184. These results accord with preclinical resistance selection studies in cultured cells where the S282T mutation took an average of over 7 weeks to appear and S282T replicon-bearing cells exhibited 5% of wild-type replication capacity. Conclusions: In 3and 14-day clinical trials with IDX184, no pre-existing S282T variant was detected. No S282T substitution or other treatment-emergent variants were detected after IDX184 treatment. These results, along with the results of in vitro preclinical studies, are consistent with the concept of a high genetic barrier to resistance for nucleoside agents such as IDX184.