Eukaryotes silence gene expression in the presence of double-stranded RNA homologous to the silenced gene. Silencing occurs by the targeted degradation of mRNA. Biochemical reactions that recapitulate this phenomenon generate RNA fragments of 21--23 nucleotides from the double-stranded RNA. These stably associate with an RNA endonuclease and probably serve as a discriminator to select mRNAs. Once selected, mRNAs are cleaved at sites 21--23 nucleotides apart. This mechanism, termed RNAi, has functional links to viral defense and silencing phenomena, such as cosuppression. It also functions to repress the hopping of transposable elements.