This study has multiple parts. It will assess the safety, tolerability and pharmacokinetics (PK) of AT-527 in healthy subjects and subjects infected with hepatitis C virus (HCV). In addition, the study will assess the antiviral activity of AT-527 in subjects infected with HCV.

Inclusion Criteria:

All subjects (healthy and HCV-infected subjects):

• Must agree to use two methods of birth control from Screening through 90 days after administration of the last dose of study drug
• Must have a negative pregnancy test at Screening and prior to dosing
• Minimum body weight of 50 kg
• Willing to comply with the study requirements and to provide written informed consent

Additional inclusion criteria for HCV-infected subjects:

• Must have not received prior treatment for HCV infection
• Documented clinical history compatible with chronic HCV infection
• Plasma HCV RNA ≥ 5.0 log10 IU/mL at Screening.

Exclusion Criteria:

All subjects (healthy and HCV-infected subjects):

• Pregnant or breastfeeding
• Infected with hepatitis B virus or HIV
• Abuse of alcohol or drugs
• Use of other investigational drugs within 28 days of dosing
• Other clinically significant medical conditions or laboratory abnormalities