PIWI-interacting RNA (piRNA) is the largest small non-coding RNA molecule expressed in animal cells and is highly enriched in the germline tissues of most metazoans. The piRNA contains 26-33 nt, slightly longer than the miRNA and siRNA, and has 2'-O-methyl modification sites at the 3' end and strong bias toward uridine at the 5' end (1 U). piRNA interacts with PIWI protein to forms an RNA-protein complex, the core of the piRNA pathway whose primary function is to silence the transposable element.