Ligand-dependent transcriptional regulators were generated by fusion of designed Cys(2)-His(2) zinc finger proteins and steroid hormone receptor ligand binding domains. To produce novel DNA binding domains, three-finger proteins binding specific 9-base pair sequences were constructed from modular building blocks. Fusion of these zinc finger proteins to a transcriptional activation domain and to modified ligand binding domains derived from either the estrogen or progesterone receptors yielded potent ligand-dependent transcriptional regulators. Together with optimized minimal promoters, these regulators provide 4-hydroxytamoxifen- or RU486-inducible expression systems with induction ratios of up to 3 orders of magnitude. These inducible expression systems are functionally independent, and each can be selectively switched on within the same cell. The potential use of zinc finger-steroid receptor fusion proteins for the regulation of natural promoters was also explored. A gene-specific six-finger protein binding an 18-base pair target sequence was converted into a ligand-dependent regulator by fusion with either two estrogen receptor ligand binding domains or one ecdysone receptor and one retinoid X receptor ligand binding domain. These single-chain receptor proteins undergo an intramolecular rearrangement, rather than intermolecular dimerization and are functional as monomers. Thus, the ability to engineer DNA binding specificities of zinc finger proteins enables the construction of ligand-dependent transcriptional regulators with potential for the regulation of virtually any desired artificial or natural promoter. It is anticipated that the novel chemically regulated gene switches described herein will find many applications in applied and basic research, where the specific modulation of gene expression can be exploited.