An Inside Job for siRNAs

Among the three main categories of small silencing RNAs in insects and mammals—siRNAs, miRNAs, and piRNAs—siRNAs were thought to arise primarily from exogenous sources, whereas miRNAs and piRNAs arise from endogenous loci. Recent work in flies and mice reveals several classes of endogenous siRNAs (endo-siRNAs) that contribute to functions previously reserved for miRNAs and piRNAs, including gene regulation and transposon suppression.