Subjects in this study were participants in one of 6 studies: (i) AIDS Link to Intravenous experience (ALIVE)21 (link) (N = 281); (ii) Multicenter Hemophilia Cohort Study (MHCS)22 (link) (N = 305); (iii) Hemophilia Growth and Development Study (HGDS)23 (link) (N = 106); (iv) Correlates of Resolved Versus Low Level Viremic Hepatitis C Infection in Blood Donors study (REVELL) (N = 85); (v) an HCV clinic cohort in Portland, Oregon (N = 51); (vi) a cohort of injection drug users from the UK (N = 180) (see Methods for details). Fifty one worldwide populations (N = 2371) from the ALlele FREquency Database (ALFRED)24 (link), were also genotyped in this study. Details of sampling and ethnographic information for these populations can be found at http://alfred.med.yale.edu/. All populations were in Hardy-Weinberg equilibrium with the exception of one, the Finnish sample (n = 33, p = 0.05). Genotyping was performed using the ABI TaqMan allelic discrimination kit and the ABI7900HT Sequence Detection System (Applied Biosystems, Foster City, CA). SAS 9.1 (SAS Institute) was used for statistical analyses.