RNA interference (RNAi), a form of post-transcriptional gene silencing induced by introduction of double-stranded RNA (dsRNA), has become a powerful experimental tool for studying gene function. The RNAi phenomenon was first discovered in Caenorhabditis elegans and is characterized by sequence-specific gene silencing elicited by introduction of dsRNA (Fire et al. 1998; Elbashir et al. 2001) complementary to a target mRNA. In the endogenous RNAi pathway, long dsRNA is cleaved by the RNase III type endonuclease, Dicer, to produce 21–23 base pair (bp) short interfering RNAs. The siRNAs are in turn unwound and incorporated into a multiprotein complex known as the RNA-induced silencing complex (RISC), generating a sequence-specific nuclease that guides the cleavage of specific complementary mRNAs. In mammalian cells, direct introduction of siRNAs is used to experimentally initiate RNAi, because introduction of long dsRNA induces a potent antiviral response in addition to RNAi.