Piwi-interacting RNAs (piRNAs) were reported in 2006 as a novel class of small 
non-coding RNAs associated with Piwi proteins of the Argonaute/Piwi family. 
Recent studies have revealed not only the biogenesis of piRNAs and their roles 
in transposon silencing, but also the function of the Piwi-piRNA pathway in 
epigenetic and post-transcriptional regulation of gene expression. In addition, 
the function of this pathway in somatic cells has also been more systematically 
characterized. The new findings reveal the Piwi-piRNA pathway as a more general 
mechanism of gene regulation.