The term RNA interference (RNAi) describes the use of double-stranded RNA to target specific mRNAs for degradation, thereby silencing their expression. RNAi is one manifestation of a broad class of RNA silencing phenomena that are found in plants, animals and fungi. The discovery of RNAi has changed our understanding of how cells guard their genomes, led to the development of new strategies for blocking gene function, and may yet yield RNA-based drugs to treat human disease.