Although we used the Malthusian parameter, M, and the basic reproduction number, R0* to compare the differences in viral characteristics here, we need to consider evasion of the acquired immune response in infected rhesus macaques to further understand the differing pathogenesis displayed by these viruses, such as systemic CD4+ T-cell depletion in SHIV-KS661 infections. This evidence concerns the gene CD4 and infection.