A Phase 3, Open-label Study to Investigate the Efficacy and Safety of Sofosbuvir Plus Ribavirin in Chronic Genotype 1, 2, 3 and 4 Hepatitis C Virus (HCV) and Human Immunodeficiency Virus (HIV) Co-infected Subjects

Inclusion Criteria:

• Age > or = 18 years with chronic HCV genotype 1, 2, 3, or 4 and co-infected with HIV-1 infection
• HCV RNA > 10,000 IU/mL at Screening
• HCV treatment-naïve for HCV genotypes 1, 2, 3, or 4
• Previous HCV treatment for HCV genotypes 2 or 3
• On a stable, protocol-approved, HIV antiretroviral (ARV) regimen with undetectable HIV-RNA for greater than 8 weeks prior to screening.
• Not currently receiving HIV ARVs
• Presence or absence of cirrhosis; a liver biopsy may be required.
• Healthy according to medical history and physical examination with the exception of HCV and HIV diagnosis.
• Agree to use two forms of highly effective contraception for the duration of the study and 6 months after the last dose of study medication.

Exclusion Criteria:

• HCV genotype 1 or 4 with previous HCV treatment
• Poor control with HIV ARV regimen requiring a possible dose modification of therapy within 4 weeks of study medication dosing
• A new AIDS-defining condition diagnosed within 30 days prior to screening
• Prior use of any other inhibitor of the HCV NS5B Polymerase.
• History of any other clinically significant chronic liver disease.
• Evidence of or history of decompensated liver disease.
• Chronic hepatitis B virus (HBV) infection.
• Hepatocellular carcinoma (HCC) or other malignancy (with exception of certain resolved skin cancers).
• Chronic use of immunosuppressive agents or immunomodulatory agents.
• Clinically-relevant drug or alcohol abuse within 12 months of screening.
• History or current evidence of any condition, therapy, laboratory abnormality or other circumstance that might confound the results of the study, or interfere with the subject's participation for the full duration of the study or not be in the best interest of the subject in the opinion of the investigator.