CD4 and HIV infectious disease: HIV infection involves an initial interaction between the viral envelope glycoprotein gp120 and the CD4 molecule on the surface of target cells, followed by a second interaction with a co-receptor; for HIV-1, the classical co-receptor function is provided by one of two different molecules, the C-C chemokine receptor 5 (CCR5) or the C-X-C chemokine receptor 4 (CXCR4), which have a normal physiological function serving as receptors for soluble chemokines [4,5].