CXCR4 and infection: HIV-1 enters target cells by binding its envelope glycoprotein gp120 to the CD4 receptor and/or co-receptors such as C-C chemokine receptor type 5 (CCR5; R5) and C-X-C chemokine receptor type 4 (CXCR4; X4), and R5-tropic viruses predominate during the early stages of infection.