PIWI-interacting RNAs (piRNAs) are endogenous small noncoding RNAs that act as 
guardians of the genome, protecting it from invasive transposable elements in 
the germline. Animals lacking piRNA functions show defects in gametogenesis and 
exhibit sterility. Their descendants are also predisposed to inheriting 
mutations. Thus, the piRNA pathway has evolved to repress transposons 
post-transcriptionally and/or transcriptionally. A growing number of studies on 
piRNAs have investigated piRNA-mediated gene silencing, including piRNA 
biogenesis. However, piRNAs remain the most enigmatic among all of the 
silencing-inducing small RNAs because of their complexity and uniqueness. 
Although piRNAs have been previously suggested to be germline-specific, recent 
studies have shown that piRNAs also play crucial roles in nongonadal cells. 
Furthermore, piRNAs have also recently been shown to have roles in 
multigenerational epigenetic phenomena in worms. The purpose of this review is 
to highlight new piRNA factors and novel insights in the piRNA world.