“…Despite extensive genomic diversity, a unifying feature of human and nonhuman primate lentiviruses is that the cell receptor is the CD4 antigen, a differentiation marker on the surface of T-helper lymphocytes. 43,44,53,58,97,107 At present, the origins of HIV-1 and HIV-2 infections remain an enigma, although some investigations have indicated that both viruses have risen from zoonotic transmissions between nonhuman primates and humans. 97 The analysis of genetic sequencing has revealed that the genomes of SIV SMM and HIV-2 exhibit a high degree of homology although SIV CPZ is most closely related to HIV-1 strain.…”