Corti et al. make safer a promising approach for gene therapy of Parkinson's disease, by creating a system to tightly control expression of the therapeutic gene (see p. 349). Gene delivery techniques have been developed that can genetically modify human neural progenitor cells, proliferate them in culture, and implant them into the CNS. Their twist was to design an adenovirus vector containing an antibiotic-responsive regulatory system that could modulate expression of the therapeutic gene—in this case, the rate-limiting enzyme in L-dopamine synthesis. After the neural cells infected with the vector were transplanted into rats, expression of the gene could be reversibly turned off by injections of the antibiotic.