Gene function during mammalian development is often studied by making irreversible changes to the genome. This approach has a major drawback in that the function of the gene in question must be deduced from the phenotype of animals that have been deficient for the product of the disrupted gene throughout ontogeny. Compensation for the loss of the gene product could yield an apparently unaltered phenotype. Alternatively, the changes in the regulation of other genes could yield a misleading phenotype. If the genetic manipulation results in embryonic or neonatal lethality, gene function at later stages of development cannot be analyzed. It would thus be highly advantageous if the expression of a particular gene could be restricted both temporally and spatially through the use of an inducible genetic system. This paper describes the various inducible genetic expression systems developed for use in mammalian cells, with particular emphasis on their application in the nervous system of transgenic mice.