CD4 and infection: These data indicate that differential intraspecies permissivity for SIV replication in rhesus monkey lymphocytes is not dependent on the activation or memory state of CD4+ T cells, is not entry dependent, does not reflect the susceptibility of a subpopulation of cells, is not dependent on multiple rounds of infection, is not specific for SIVsmE660 and is manifested at a stage of viral replication prior to viral gene expression.