It was observed more than a decade ago that not all HIV-1 variants are capable of establishing new infection upon transmission: while both CCR5- tropic (R5) and CXCR4-tropic (X4) HIV-1 variants are present in various body fluids, only R5 viruses are apparently transmitted and dominate the early stages of HIV-1 disease [11]. Here, CXCR4 is linked to infection.