Of the wide variety of regulatory systems that have been developed to control gene expression in higher eukaryotes, those utilizing elements from prokaryotic systems presently achieve the highest specificity. In particular, systems based on the repressor/operator elements of an Escherichia coli tetracycline resistance operon appear broadly applicable. During the past year, the usefulness of these systems has been demonstrated not only at the cellular level, but also at the organismal level (i.e. in transgenic animals and plants).