RNA interference (RNAi ) by double stranded RNA (dsRNAs) molecules of approximately 20-25 nucleotides termed short interfering (siRNAs) is a powerful method for preventing the expression of a particular gene. The dsRNA dominantly silences gene expression in a sequence-specific manner by causing the corresponding endogenous mRNA to be degraded. The technique was first developed in Caenorhabditis elegans, and was rapidly applied to a wide range of organisms. Methods for expressing siRNAs in cells in culture and in vivo using viral vectors, and for transfecting cells with synthetic siRNAs, have been developed and are being used to establish the functions of specific proteins in various cell types and organisms. For example, chemically synthesized or in vitro transcribed siRNAs can be transfected into cells, injected into mice, or introduced into plants. siRNAs can also be expressed endogenously from siRNA expression vectors or PCR products in cells or in transgenic animals.