CD4 and HIV infectious disease: To address this gap in knowledge we used (i) HIV infection, (ii) variations in the genes encoding CC chemokine receptor 5 (CCR5), the major coreceptor for HIV-1 [1], and CC chemokine ligand 3-like 1 (CCL3L1), the most potent CCR5 agonist and HIV-suppressive chemokine [2], [3], [4], [5], and (iii) the laboratory markers (CD4+ T cell count and viral load) currently used to evaluate HIV-infected patients, as a model system.