Expression of Bet and Tas in FV susceptible and restricted cell lines. FV susceptible cell lines containing abundant concentrations of Bet and low concentrations of Tas are still able to enhance the LTR and internal promoter (IP) (panel A). Restricted FV cell lines are associated with reduced LTR and IP activity (panel B). Bet-mediated inhibition of IP activation results in reduced concentrations of Tas and consequently further inhibition of IP activity. The underlying mechanism of Bet-mediated inhibition of IP could be a negative control on transcription or translation of the Tas gene as indicated by a question mark.