All GWAS participants came from the Urban Health Study (UHS): a serial, cross-sectional sero-epidemiological study conducted from 1986 - 2005. Eligibility criteria for initial entry to the UHS were: (1) injection drug use in past 30 days; (2) ability to provide informed consent; and (3) age 18 or older. Additional criteria for the GWAS were adequate stored serum volume (≥1.5ml), valid HIV test results, and self-identification as African American or European American.

Cases were identified as those with a positive HIV test. Controls were identified as those with a negative HIV test. Among the available controls participants were selected for genotyping by frequency matching to cases on: (1) self-reported ancestry; (2) sex; (3) age; (4) year of ascertainment; and (5) HIV risk class.