RNA interference (RNAi) is a useful tool for degrading targeted messenger RNAs (mRNAs) and thus "knocking down" the abundance of the encoded protein. We have been using RNAi in cultured Drosophila cells to evaluate the effect of "knocking down" numerous mRNA processing factors on the alternative splicing of specific pre-mRNAs. This relatively simple technique has allowed us to identify a number of splicing factors that impact the alternative splicing of particular alternatively spliced exons. This approach can be extended to examine the splicing of nearly any gene.