The researchers analyzed information from previous studies that had tested for associations between genetic alleles and the course of HIV-1 infection. The studies had been conducted by 19 groups of researchers in the United States, Europe, and Australia. In each study, the authors periodically checked a group of people with HIV-1 infection to see who developed AIDS or died. All study participants had had special genetic testing to determine whether or not they carried particular alleles (CCR5-Δ 32, CCR2-64I, SDF-1 3′A) that have been suspected to affect progression of HIV infection. The authors combined the results from all of the studies in a special way (individual-patient–based meta-analysis) to see which alleles were associated with AIDS or death.