CD4 and infection: For example, CMMT/CD4 cells, rhesus mammary gland tumor cells engineered to express human CD4, are susceptible to efficient infection by SIVSMM, SIVMAC, and SIVAGM (SIV originating from one of the African green monkey species) (146) and HIV-2 (147) but are not susceptible to infection by HIV-1 (147) (Table 1; Fig. 1c).