We have shown that macrophages isolated from normal human small intestine are highly refractory to productive HIV-1 infection [13], [14], [15], supporting observations that memory CD4+ T cells rather than macrophages are the predominant mononuclear target cell in the intestinal mucosa during primary HIV-1 infection [19], [20], [21], [22], [23], [24], [25], [26]. This evidence concerns the gene CD4 and HIV-1 infection.