CD4 and viral infectious disease: Given the much lower virus infection rate obtained with SIVmac239 and HIV-2 compared to HIV-1 (Fig. S1), and to conduct mechanistic studies, we generated a CD4- and CCR5-expressing (Fig. S2) human monocyte THP-1 cell line, which, upon differentiation into macrophage-like cells, can be efficiently infected by either HIV-1, HIV-2, or SIVmac (Fig. S3).