“…Friend spleen focus-forming virus (F-SFFV) is a highly pathogenic, replication-defective retrovirus inducing acute erythroleukaemia in mice which is either associated with a polycythaemia (F-SFFVp) or a slight anaemia (for reviews, see Ruscetti & Wolff, 1984;Ostertag et al, 1987;Kabat, 1989). In both instances, the presence and expression of the viral envelope (env) gene was shown to be essential and sufficient for induction of the disease (Linemeyer et al, 1981(Linemeyer et al, , 1982Machida et al, 1985;Wolff & Ruscetti, 1985Li et al, 1986).…”