Regulated gene expression systems for the study of gene function in prokaryotes and yeast have been available for several years. However, comparable systems in higher organisms are more complex and problematic. Recently, regulatory proteins from distant species have been used to establish highly specific control systems in eukaryotic cells. This is possible because their action can be modulated by effectors that are inert to the physiology of the organism or cell and therefore do not elicit pleiotropic effects. Such monospecific regulatory circuits open up new possibilities for the study of gene function in vivo.