CD4 and HIV-1 infection: Cell surface CD4 levels can be down-regulated by exposure to gangliosides[20],soluble HIV-1 gp120 [21], phorbol esters [17], [22] and during HIV-1 infection [23], [24].Moreover, down-regulation of viral receptors is a common mechanism used by mostretroviruses to avoid superinfection (multiple rounds of infection) and to promoteviral release.