RNA interference (RNAi) is the sequence-specific silencing of gene expression in response to double-stranded (ds) RNA. Biochemical and genetic studies have begun to provide the molecular details for how dsRNA can lead to the targeted disruption and silencing of gene expression. In the first step of the RNAi pathway, long dsRNA is cleaved into siRNAs. The antisense strand of the siRNA guides the endonuclease activity of RISC to the homologous site on the cognate mRNA, resulting in the cleavage of the mRNA in the center of the siRNA–mRNA recognition site. The binding of long dsRNA leads to the activation of the dsRNA-dependent protein kinase (PKR). The demonstration that chemically or enzymatically synthesized siRNAs are capable of effectively silencing gene expression has led to the rapid development of RNAi-based technologies. These technologies have served to address two limitations inherent in the gene silencing by transfection of synthetic siRNAs, the transient nature of the silencing phenotype and the low efficiency of siRNA transfection in certain cell types, particularly primary cells
