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A 32–nucleotide deletion (δ32) within the β–chemokine receptor 5 (CCR5) gene has been described in subjects who remain uninfected despite extensive exposure to HIV–1. This allele was found to be common in the Caucasian population with a frequency of 0.0808, but was not found in people of African or Asian ancestry. To determine its role in HIV–1 transmission and disease progression, we analyzed the CCR5 genotype of 1252 homosexual men enrolled in the Chicago component of the Multicenter AIDS Cohort Study (MACS). No infected participant was found to be homozygous for the 32 allele, whereas 3.6% of at–risk but uninfected Caucasian participants were homozygous, showing the highly protective role of this genotype against sexual acquisition of HIV–1. No evidence was found to suggest that heterozygotes were protected against HIV–1 infection, but a limited protective role against disease …