to be assessed in an identical genetic background. Regulated gene expression is also gaining increasing importance for biotechnological applications since it allows conditional metabolic engineering and achievement of specific cell culture states in a timely manner (Fussenegger et al., 1997a; Fussenegger et al., 1998a) . For example, regulated metabolic engineering in a multicistronic configuration allows differentiation of a cell culture process into two stages: a nonproductive growth phase in which the cells are rapidly expanded to the desired cell density, and a subsequent non-proliferating production phase where the cells can devote all of their metabolic capabilities to the production of protein instead of biomass.