Several in vivo regulated eukaryotic promoters have been described (Schweinfest et al., 1988; Israel and Kaufman, 1989; Ko et al., 1989; Hu and Davidson; Mattioni et al., 1994) and used for regulated gene expression. However, as most of these regulated promoters are derived from regulatory circuits which mediate metabolic responses, the corresponding regulating external stimuli may lead to undesired pleiotropic effects. More successful transcriptional regulation circuits rely on basic regulatory machineries of heterologous origin which are genetically adapted for use in mammalian cells. Besides the lac switch (Fieck et al., 1992) and the ecdysoneresponsive system , the tetracyclineregulatable expression system (tet system; Gossen and Bujard, 1992) is by far the most popular. The tet system consists of two separate genetic entities, the tet-responsive transactivator (tTA) and the tTA responsive promoter, P hCMV * −1 each of which represents a chimeric genetic configuration composed, respectively, of a protein fusion between the bacterial tet repressor and the VP16 domain of the herpes simplex virus (tTA) and a genetic fusion which places a tet operator adjacent to a minimal cmV promoter (P hCMV * −1 ). While the bacterial parts are responsible for promoter recognition and integrate responsiveness to the external stimulus tetracycline, the viral parts initiate transcription (Gossen and Bujard, 1992) .