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.