The treatment of chronic hepatitis C has evolved dramatically in recent years. Without incorporation of interferon, treatments with direct-acting antivirals (DAAs) have been developed. In 2014, the first DAAs regimen licensed in Japan was 24-week combination therapy with daclatasvir (DCV) and asunaprevir (ASV) for genotype 1 chronic hepatitis C: DCV, a NS5A replication complex inhibitor with pan-genomic activity, ASV, a NS3 protease inhibitor. Our Kyushu University Liver Disease Study (KULDS) group brought real life experience from a multicenter cohort that sustained virological response (SVR) rate after DCV plus ASV therapy was 90.3%. This therapy is valuable for interferon-ineligible / intolerant patients such as those with depression, cirrhosis, or advanced age as well as pervious interferon-based regimen non-responders. However, single resistance-associated substitution (RAS) of hepatitis C virus genome is observed in DAA-naïve patients to some extent: 22.3% in our KULDS cohort. The virological response after DCV plus ASV therapy was extremely influenced by the RASs: extremely low SVR rate of 35.3% for our patients with baseline RASs. Moreover, treatment failure induces the emergence of highly resistant RASs. In addition, many of the RASs in NS5A may be cross-resistant to other DAAs of the same class. The combination regimens of 2 or 3 classes of DAAs are the mainstay of treatment for genotype 1 chronic hepatitis C. Of these classes of DAAs, NS5A inhibitor represents the key class included in the majority of combination therapy. Evidence for the efficacy in the retreatment of patients who failed prior treatment with regimens that include NS5A inhibitor was lacking. However, recently, reduction in efficacy of retreatment with ledipasvir (LDV) plus sofosbuvir (SOF) after DCV plus ASV failure was observed in several studies in Japan: low SVR rate of 64-87%. Therefore, current EASL guidelines do not recommend the use of NS5A inhibitors for retreatment in patients found to have NS5A RAS. AASLD/IDSA and JSH guidelines recommend testing for RASs. Newer DAAs with high potency may provide better therapeutic effect in patients with identified RASs in the near future.