RNA interference is a process of gene silencing that plays an important role in development and maintenance of the genome. The RNAi pathway is complex. It is initiated by the enyzme dicer which cleaves double stranded RNA (dsRNA) into 20-25 bp fragments. An RNA-induced silencing complex (RISC) is then formed by base pairing between complementary mRNA and 1 of the 2 strands of each new fragment. This formation of the RISC complex is followed by degradation of the complementary mRNA by the endonuclease argonaute. Argonaute is the catalytic component of the complex. The short 20-25 bp fragments are known as small interfering RNA (siRNA) when they are artificially introduced and microRNA (miRNA) when they are produced endogenously. RNA interference has become a valuable research tool since it allows the prevention of translation of specific genes by introducing siRNA complementary to the mRNA one wishes to suppress.

In plants, Drosophila, and C elegans, RNA interference plays an important role in viral immunity. The plant Arabidopsis expresses several dicer homologs that react to different viruses. Some plants also express siRNA in response to invasion by bacteria. The role of RNAi in mammalian viral immunity is not as well understood. In addition to viral immunity, components of the RNAi pathway play a role in the maintenance of the genome, pretranscriptionally. RNA-induced transcriptional silencing (RITS) is a process by which argonaute and other proteins act to modify histones and induce heterochromatin formation. This serves to downregulate genes by preventing transcription. Endogenously produced miRNA regulates development by translational repression. These miRNA are especially important in the timing of morphogenesis and maintaining stem cells in an undifferentiated state.

In addition to research applications, RNAi may play an important future role in medicine. It is problematic to deliver long dsRNA strands into mammals due to the interferon response. The use of siRNA mimics has met with more success. Sirna Therapeutics and Alnylam Pharmaceuticals were the first to introduce siRNA into clinical trials for the treatment of macular degeneration and respiratory syncytial virus, respectively. Other clinical applications for RNAi may include antiviral activity, treatment of cancer, and neurodegenerative diseases such as Huntington's disease. An important area of research for RNAi therapy is the effective delivery of siRNA to target cells.