Specific cytotoxicity against Env-expressing cells was demonstrated in two types of in vitro systems: a) direct killing assays, in which Env-expressing cells (either stable transfectants or constitutively HIV-infected cell lines) were potently killed in dose-dependent fashion, whereas the corresponding parental cells lacking Env were unaffected [16], [26]–[28], and b) spreading infection inhibition assays, in which infectious HIV-1 is added to permissive target cells, and virus production is measured (p24 or reverse transcriptase) [27], [29]–[32]. Here, ERVW-1 is linked to infection.