RNA interference (RNAi) is a biological process in which the introduction of double-stranded RNA (dsRNA) into a cell leads to targeted post-transcriptional gene silencing. Historically, RNAi has been used as a tool for functional genomics research in C.elegans and Drosophila. Initial attempts to activate the RNAi pathway in mammalian cells were unsuccessful, since the introduction of dsRNA＞30 nucleotides in length results in non-specific suppression of gene expression. Much of this response is due to activation of the dsRNA-dependent protein kinase PKR, and the subsequent phosphorylation and inactivation of the translation factor elF2a. As RNAi mechanism being extensively studied, researchers discovered that double stranded short interfering RNA (siRNA) oligotides of 21～23 nucleotides could be used to mediate a gene silencing effect in mammalian cells. The application of RNAi to mammalian cells has the potential to revolutionize the field of functional genomics. The ability to simply, effectively, and specifically down-regulate the expression of genes in mammalian cells holds enormous scientific, commercial, and therapeutic potential.