CD4 and HIV-1 infection: The interaction of the human immunodeficiency virus type 1 (HIV-1) envelope protein (Env) with its cellular receptor CD4 [1] was recognized early as an opportunity to selectively inhibit HIV-1 infection by competition with soluble CD4 or by targeting HIV-1 infected cells with cytocidal molecular conjugates of CD4 such as CD4-Pseudomonas exotoxin [2,3].