The transfer of pro-apoptotic genes to tumors is one of the most promising strategies for anticancer gene therapy. However, the use of potentially toxic genes, such as tumor suppressor genes or apoptotic genes, needs controllable transgene activation. To achieve regulation of the transgene at a desired time, we developed an adenovirus (Ad) vector, in which the apoptotic activity of the target gene has been made 4-OHT-dependent by fusion to the ligand binding-domain of the estrogen receptor (ER). For evaluation of the system in human tumor cells, we used the E2F1 gene which encodes a transcription factor that triggers massive apoptosis in several human cancers. AdER-E2F1 expressed high levels of transgene over at least 1 week. Upon activation of E2F1 by the ligand 4-hydroxy-tamoxifen (4-OHT) the ER-E2F1 fusion protein correctly translocated from the cytosol to the nucleus, transactivated E2F-dependent promoters, and rapidly induced substantial E2F1-related toxicity. Finally, experiments in nude mice showed tightly regulated tumor growth suppression in vivo. Taken together, our system represents a powerful approach for tight regulation and rapid induction of cytotoxicity as the major criteria for safe gene delivery.