RNA interference (RNAi) can be used to silence genes in a number of taxa, including plants, nematodes, protozoans, flies, and mammals represented by mouse embryos and cultured mammalian cells. To investigate signal transduction pathways, we used RNAi on Drosophila-cultured cells, which affords the opportunity to study protein function in a simple, well-defined cell culture system. Furthermore, the results obtained from experiments performed on cultured cells can be confirmed and extended in the whole organism, which, in the case of Drosophila, is also RNAi responsive. RNAi takes advantage of the unique ability of double-stranded RNA (dsRNA) molecules to induce posttranscriptional gene silencing in a highly specific manner. This silencing is efficacious and long-lived, as it is passed to subsequent generations in insect cell culture. To date, all Drosophila cell lines tested (S2, KC, BG2-C6, and Shi) respond to dsRNAs by ablating expression of the target protein. Furthermore, all dsRNAs tested (more than 15) have been effective at silencing the target gene. Drosophila cell cultures are simple, easily manipulated model systems that will facilitate loss-of-function studies applicable to a wide variety of questions.