Stromal-derived factor (SDF-1) is the principal ligand for CXCR4, a coreceptor with CD4 for T lymphocyte cell line–tropic human immunodeficiency virus–type 1 (HIV-1). A common polymorphism,SDF1-3′A, was identified in an evolutionarily conserved segment of the 3′ untranslated region of the SDF-1 structural gene transcript. In the homozygous state, SDF1-3′A/3′Adelays the onset of acquired immunodeficiency syndrome (AIDS), according to a genetic association analysis of 2857 patients enrolled in five AIDS cohort studies. The recessive protective effect ofSDF1-3′A was increasingly pronounced in individuals infected with HIV-1 for longer periods, was twice as strong as the dominant genetic restriction of AIDS conferred by CCR5 andCCR2 chemokine receptor variants in these populations, and was complementary with these mutations in delaying the onset of AIDS.