The Regulated Mammalian Expression System is designed to allow tight control of gene expression in mammalian cells. The system is based on a novel on/off switch that relies on the rapid and sensitive modulation of a chimeric transactivator protein by coumerin-related compounds. Nanomolar concentrations of the antibiotic coumermycin (i.e., coumermycin A1) promote homodimerization of a chimeric transactivator resulting in activation and binding to operator sequences located upstream of a minimal promoter that drives transcription of the coding sequence of interest. The levels of protein expression can be regulated by adjusting the coumermycin concentration. The expression can be rapidly switched off by adding novobiocin, which acts as an antagonist by dissociating the dimerized transactivator protein, inhibiting expression.