For a number of organisms, including plants, worms, flies, and mice, injection of double-stranded (ds) RNA has been shown to silence the corresponding gene by specific degradation of the transcript. However, the method has been of limited value in the analysis of gene function because the transient nature of the dsRNA interference (RNAi) makes it unsuitable for the study of older individuals. On page 896, Kennerdell and Carthew describe a way of overcoming this problem in Drosophila melanogaster by expressing dsRNA from a transgene under control of a regulatable promoter, which they introduced by P-element transposons or injection. Expression of the hairpin form of RNAi effectively silenced expression of lacZ in late-stage fly larvae.