GENERAL MECHANISMS OF RNA SILENCING

RNA interference (RNAi) was originally discovered as an artificial process of silencing cellular mRNAs in response to the addition of exogenous double-stranded RNAs (Fire et al. 1998). The power of this response strongly hinted at an endogenous function for such mech-anisms, and work in the field quickly revealed that multi-ple, RNAi-related pathways formed a previously unrecognized layer of gene regulation and genome defense that is essential to virtually all eukaryotic organ-isms. The development of methods to characterize small RNA sequences opened previously unseen worlds of genomic output in organisms ranging from plants and fungi to protozoans and mammals. These fell into a num-ber of classes based on their biogenesis mechanisms, genomic origins, and biological functions. Thus far, smal