With the explosion of genomic information, there is an increasing need to analyze gene function in a high-throughput fashion. This makes reverse genetic approaches extremely attractive; however, in most mammalian and vertebrate systems it has been difficult and time-consuming to develop a cellular model deficient in one or more proteins. The discovery of RNA interference (RNAi) has made loss-of-function studies relatively quick and easy and amenable to high-throughput formats (1 –3 ). Double-stranded RNAs (dsRNAs) 21 nucleotides in length, known as small interfering RNAs (siRNAs), are introduced into the cytosol, where they are unwound (4 ), allowing the antisense strand to interact in a sequence-specific manner with the complementary mRNA (5 ). Binding of the antisense strand to the target mRNA triggers cleavage of the mRNA (6 ). siRNA-mediated gene silencing is very specific, most likely because of the high specificity of nucleotide base-pairing.