CXCR4 and infection: During subtype B HIV-1 (B-HIV) infection, viruses that use CCR5 as the coreceptor for HIV-1 entry (R5 strains) predominate at early stages of infection, but viral variants that have acquired the ability to use CXCR4 instead of CCR5 (X4 strains) or together with CCR5 (R5X4 strains) emerge in 40 to 50% of subjects and accelerate the rate of disease progression [14], [15].