In the case of vector ACE-GFP replicating in NIH-3T3 cells, the ratio of env to transgene signal begins to increase at infection cycle 9, but does not increase at all in HEK293 or U87-MG cells infected with ACE-GFP (Fig. 2), indicating that no major deletion mutants emerge in these cells. Here, ERVW-1 is linked to infection.