Small RNAs just got bigger: Piwi-interacting RNAs (piRNAs) in mammalian testes.

Small RNAs constitute a large family of regulatory molecules with diverse functions in eukaryotes. Hallmarks of small RNAs are their dependence on double-stranded RNAs (dsRNA)-specific RNase III-type enzymes for biogenesis and their association with Argonaute family proteins for the silencing process. At least two classes of small RNAs have previously been described: microRNAs (miRNAs) derived from hairpin-shaped precursors and small interfering RNAs (siRNAs) generated from long dsRNAs. Recent articles reported a novel class of small RNAs that are expressed specifically and abundantly in the spermatogenic cells of mice. These RNAs are bigger (26-31 nucleotides [nt]) than most previously described small RNAs (21-23 nt) and are associated with Piwi-subclade members of the Argonaute protein family. Although the biogenesis and function of these RNAs are yet to be determined, these findings may add new dimensions in small RNA biology and germline cell biology.