CD4 and viral infectious disease: Together with the fact that HIV-1 infection of TZM-bl cells (epithelial-like cells expressing CD4, CXCR4 and CCR5) also results in the assembly and secretion of infectious viral progeny, our data suggests that oral and vaginal epithelial cells are able to support productive viral infection, but only if HIV-1 gains entry into the cell through non-conventional (endocytic) mechanisms.