While a co-infection in vitro study with more fit subtype B and less fit subtype C viruses indicates more fitness of subtype B over subtype C viruses [40], several in-vivo infection studies on rhesus macaques have shown that HIV-1 subtype C env chimeric viruses demonstrate greatly enhanced infectivity [41-43] and replication efficiency as compared to subtype B and E viruses [44,45]. This evidence concerns the gene ERVW-1 and infection.