CXCR4 and AIDS: Progression of HIV-1 infection from early, asymptomatic stages of disease to acquired immunodeficiency syndrome (AIDS) is associated with a switch in viral coreceptor specificity from CCR5-using (R5) viral strains to those able to use CXCR4 (X4) or both coreceptors (R5X4) in 40–50% of infected adults [4-8] (reviewed in [9]).