PIWI interacting RNAs (piRNAs) are a subset of small RNAs predominantly expressed in the germlines of various species. In Drosophila, the main sources of piRNAs are transcripts of mobile DNA elements, including active transposons and their wreckage, found on the genome. After maturing from the primary transcripts, piRNAs are specifically loaded onto germline-specific Argonaute proteins - Argonaute3, Aubergine and Piwi - collectively referred to as PIWI proteins. Loss of function of PIWI proteins and/or the piRNA loci on the genome lead to derepression of transposons and causes severe defects in gametogenesis and fertility. The necessity for both PIWI proteins and piRNAs in protecting the genome of the gametes from mischievous mobile genomic elements is thus obvious. There have been extensive biochemical and genetic studies on PIWI proteins and piRNAs. These have shed light not only on the molecular mechanisms of gene silencing mediated by piRNAs and PIWI proteins, but also on their intriguing relationship with cellular genes that have been shown to be important for gametogenesis and fertility.

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