CXCR4 and HIV-1 infection: The third variable region (V3 loop) of the HIV-1 envelope protein (gp120) plays an important role in HIV-1 infection [1,2], being the primary determinant for binding to one of the two 7-transmembrane receptors CCR5 (R5-tropic) and CXCR4 (X4-tropic) [3,4].