For example, a highly pathogenic SHIV strain, SHIV-KS661, which has the env gene of HIV-1 89.6 and predominantly uses CXCR4 as the secondary receptor for infection, causes an infection that systemically depletes the CD4+ T-cells of rhesus macaques within 4 weeks following infection [4, 5]. This evidence concerns the gene CXCR4 and infection.