ERVW-1 and infection: Such alternative entry mechanisms seem to be particularly important for P-MLVs, viruses that are less able to establish effective superinfection immunity against further infection [99, 108] either because they may have lower binding affinity for the XPR1 receptor than the X-MLVs or because Env-bound receptors may recycle rapidly into acidic compartments where the Env-receptor complex is disrupted allowing the freed receptor to recycle back to the plasma membrane.