One of the highly pathogenic SHIV strains, SHIV-KS661, which has the env gene of HIV-1 89.6 and predominantly uses CXCR4 as the secondary receptor for its infection [2], causes an infection that systemically depletes the CD4+ T cells of rhesus macaques within 4 weeks after infection [6,7]. The gene discussed is CD4; the disease is infection.