Here, we describe a system for the exogenous control of gene expression in mammalian cells that relies on the control of translational termination. To achieve gene regulation, we modified protein-coding sequences by introduction of a translational termination codon just downstream from the initiator AUG codon. Translation of the resulting mRNA leads to potent reduction in expression of the desired gene product. Expression of the gene product can be controlled by treating cells that express the mRNA with either aminoglycoside antibiotics or several nonantibiotic compounds. We show that the extent of regulation of gene expression can be substantial (60-fold) and that regulation can be achieved in the case of a variety of different genes, in different cultured cell lines and in primary cells in vivo. This gene regulation strategy offers significant advantages over existing methods for controlling gene expression and should have both immediate experimental application and possible clinical application.