It has been shown that viruses using CCR5 (R5-tropic viruses) are almost exclusively present during the early asymptomatic stage of the infection whereas CXCR4-using viruses (X4-tropic viruses) emerge in later phases of the infection in about 50% of cases and are associated with a CD4+ T-cell decline and progression towards AIDS [7,8]. Here, CXCR4 is linked to infection.