piRNAs—the ancient hunters of genome invaders: Figure 1.

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Abstract
In addition to miRNAs and siRNAs, a third small RNA silencing system has been uncovered that prevents the spreading of selfish genetic elements. Production of the Piwi-associated RNAs (piRNAs), which mediate the silencing activity in this pathway, is initiated at a few master control regions within the genome. The nature of the primary piRNA-generating transcript is still unknown, but RNA interference (RNAi)-like cleavage events are likely defining the 5′-ends of mature piRNAs. We summarize the recent literature on piRNA biogenesis and function with an emphasis on work in Drosophila, where genetics and biochemistry have met very successfully.