With the tetracycline (Tc)-controlled gene expression system (Tet system), an experimental approach has been developed that permits alteration of individual gene activities in complex genetic systems in a truly conditional fashion, i.e., it is temporally defined, quantitative, and reversible. Monitoring the phenotypic changes that accompany such highly defined alterations has provided new insights into numerous biologic processes hereto not amenable to genetic dissection. The various versions of the Tet system (1–3) were successfully incorporated into a variety of model organisms such as Streptomyces cerevisiae, Dictyostelium, Drosophila, Arabidopsis, moss, mice, and rats (for review, see refs. 3–5).