While Env chimeras containing Env E1 V1/V2 loops as well as the C2, V3, C3, V4, C4 and C5 regions failed to exhibit the increased infection phenotype (S2 Fig), the Env E1 gp41 region when engrafted into the Env NE1 fully recapitulated this phenotype (Fig 3A). Here, ERVW-1 is linked to infection.