In vivo electroporation is a powerful technique for the introduction of genes into organisms. Temporal and spatial regulation of expression of introduced genes, or of RNAi, would further enhance the utility of this method. Here we demonstrate conditional regulation of gene expression from electroporated plasmids in the postnatal rat retina and the embryonic mouse brain. For temporal regulation, Cre/loxP-mediated inducible expression vectors were used in combination with a vector expressing a conditionally active form of Cre recombinase, which is activated by 4-hydroxytamoxifen. Onset of gene expression was regulated by the timing of 4-hydroxytamoxifen administration. For spatial regulation, transgenes were expressed by using promoters specific for rod photoreceptors, bipolar cells, amacrine cells, Mü ller glia or progenitor cells. Combinations of these constructs will facilitate a variety of experiments, including cell-typespecific gene misexpression, conditional RNAi, and fate mapping of progenitor and precursor cells.G ain-of-function and loss-of-function studies using transgenic animals have greatly advanced our understanding of the molecular and cellular mechanisms of development and disease. In particular, conditional gene activation and inactivation have been powerful methods for studies of gene function and for the labeling and manipulation of specific cell populations in vivo (1, 2). Site-specific recombination systems (Cre/loxP and Flp/FRT) are widely used to control gene expression in transgenic mice. By crossing two transgenic lines, one expressing Cre (or Flp) under tissue-specific and/or inducible control and the other carrying two loxP (or FRT) sites, DNA recombination can lead to inducible gene expression or loss of gene expression in restricted tissues at specific times (3). Although such strategies are very useful, they are time-consuming and cannot be applied to species that are difficult to manipulate genetically.In vivo electroporation is a convenient technique for the introduction of genes into a variety of animals, including mouse, rat, and chick (4, 5). We previously reported that plasmid DNAs can be easily delivered to developing mouse/rat retinas by in vivo electroporation (6). The plasmid DNAs, electroporated from the scleral side of the postnatal day 0 (P0) retina, are preferentially introduced into retinal progenitor/precursor cells, which give rise to four different cell types; rod photoreceptors in the outer nuclear layer (ONL), bipolar and amacrine cells in the inner nuclear layer (INL), and Müller glial cells, which extend radial processes spanning the entire thickness of the retina [see supporting information (SI) Fig. 7]. The efficiency of electroporation into the developing postnatal retina is quite good, and transgene expression persists at least for a few months. Moreover, compared with other gene transfer methods, such as viral vectors, in vivo electroporation has several advantages. First, various types of DNA constructs, including RNAi vectors, are readily introduced to th...