A potent NS5A inhibitor (EC50 = 2.2 pM in Huh7 human liver cancer cells after 48 hours); inhibits the assembly of new viral replicase complexes (50 pM); exhibits activity against a broad range of HCV genotypes (EC50s = 0.0002-34 nM in Huh7 cells expressing HCV genotype 4 replicons of NS5A and EC50 = <0.2 nM for replicons from all other genotypes except 2b)