It has been reported that HIV-1 envelope glycoprotein (Env) functions, such as tropism or fusogenicity are linked to in vitro HIV-1 cytopathicity (23, 24) and viral fitness (25, 26), as well as to clinical progression of HIV-1 infection (27) and simian immunodeficiency virus (SIV) infection in vivo (28). This evidence concerns the gene ERVW-1 and HIV-1 infection.