For example, lymphocyte function-associated antigen-1 (LFA-1) regulates HIV-1 mediated-cell fusion but not viral transmission (Pantaleo et al., 1991), and E-MLV Env mutants containing amino acid substitutions at the R peptide cleavage site do not induce infection but mediate syncytium formation in XC cells (Kubo and Amanuma, 2003). This evidence concerns the gene ERVW-1 and infection.