All subjects in this study have previously been in an Idenix HCV study and received study drug for 3 consecutive days. Subjects who had received placebo in a previous Idenix study will not be enrolled in this study.

In this study, researchers will try to find answers to these questions:

• How much (if any) hepatitis C virus is in your blood after stopping your Idenix study drug?
• Is your hepatitis C virus possibly resistant to treatment with the Idenix study drug or similar drugs?

The data obtained from this study will be used to further understand the long-term efficacy of Idenix DAAs used to treat HCV infection and to further understand HCV resistance to Idenix DAAs.

Inclusion Criteria:

• Read and signed the written informed consent form (ICF) after the nature of the study has been fully explained
• Have participated in an Idenix-sponsored study of an Idenix DAA
• Received at least 3 consecutive days of DAA treatment in an Idenix-sponsored study
• Agreed to comply with the visit schedule and laboratory tests

Exclusion Criteria:

• Treatment with placebo only, in an Idenix sponsored study
• Antiviral treatment for HCV after participation in an Idenix sponsored study of an Idenix DAA

The population will include subjects who participated in studies of Idenix anti-HCV DAAs.