ABSTRACT
PIWI-interacting RNAs (piRNAs) are a class of small non-coding RNAs of 24－32 nucleotides (nt) in length, which were first found in 2006. The piRNAs in germ cells interact with reproduction-specific PIWI proteins and play an important role in the development of the reproductive system. In the reproductive system of the male mouse, piRNAs arise from long single stranded precursor transcripts expressed in discrete genomic regions. The productions of piRNAs are termed primary piRNA biogenesis and secondary piRNA biogenesis. Mature piRNAs can silence the genome instability threat posed by mobile genetic elements called transposons. Furthermore, piRNAs regulate the expression of mRNAs transcriptionally and post-transcriptionally in spermiogenesis. This review summarizes the biogenesis and key functions of piRNAs in the reproductive system of male mice.