R5 HIV-1 strains isolated from patients with AIDS (hereafter referred to as AIDS R5 (A-R5) viruses) have enhanced macrophage (M)-tropism [34-36] and cause increased levels of CD4+ T-cell death [37] compared with R5 HIV-1 strains isolated from asymptomatic individuals (hereafter referred to as pre-AIDS R5 (PA-R5) viruses). The gene discussed is CD4; the disease is AIDS.