Introduction

RNA interference (RNAi) is a gene-silencing technique first described by Fire and colleagues in Caenorhabditis elegans (Fire et al., 1998). They found that the introduction of long dsRNA into the nematode C. elegans led to the targeted disruption of the homologous mRNA. RNAi is related to a variety of other gene-silencing phenomena including posttranscriptional gene silencing (PTGS) in plants and quelling in the fungus Neurospora crassa (Jorgensen, 1990; Romano and Macino, 1992). The link between these seemingly divergent processes is the presence of dsRNA homologous to the silenced gene (Bernstein et al., 2001b; Waterhouse et al., 2001).

The addition of long dsRNAs into Drosophila melanogaster embryo extracts was shown to silence gene expression in a sequence-specific manner, recapturing the RNAi reaction in vitro (Tuschl et al., 1999). Although initiated by long dsRNA, the effector molecules that guide the mRNA degradation were found to be small (21- to 23-nt) dsRNA species, termed small interfering (si)RNAs, produced by the cleavage of the long dsRNAs (Zamore et al., 2000). The introduction of chemically synthesized siRNAs into the extracts was found to be sufficient for targeting mRNA degradation (Elbashir et al., 2001a). Similarly, small RNA species were found in vivo in a wide variety of organisms and cells undergoing dsRNA-mediated gene silencing including plants, Drosophila embryos, Drosophila Schneider 2 (S2) cells and C. elegans (Hamilton and Baulcombe, 1999; Hammond et al., 2000; Parrish et al., 2000, Yang et al., 2001).