Small silencing RNAs: an expanding universe

Small RNAs — including miRNAs, siRNAs and piRNAs — differ in their biogenesis, modes of target regulation and biological functions. There are also interconnections between these pathways, which compete and collaborate in some of their regulatory and protective roles. Since the discovery in 1993 of the first small silencing RNA, a dizzying number of small RNA classes have been identified, including microRNAs (miRNAs), small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs). These classes differ in their biogenesis, their modes of target regulation and in the biological pathways they regulate. There is a growing realization that, despite their differences, these distinct small RNA pathways are interconnected, and that small RNA pathways compete and collaborate as they regulate genes and protect the genome from external and internal threats.