An essential role for PNLDC1 in piRNA 3＇ end trimming and male fertility in mice

Dear Editor, PIWI-interacting RNAs （piRNAs） are germ cell-specific small non-coding RNAs that are essential for silenc- ing transposable elements. Substantial efforts in the past decade have led to an understanding of how piRNAs are made. Primary piRNA biogenesis is initiated with transcription of piRNA precursors, followed by cleavage into piRNA intermediates, and finally, maturation by 3＇ end trimming and 2＇-O-methylation. Secondary piRNA biogenesis occurs through an amplification loop （ping pong pathway）; the piRNA pools generated through pri- mary processing guide MILI protein to cleave the target RNA for piRNA generation in a feed-forward loop that accelerates production of the piRNAs. Papi/Tdrkh has been implicated in processing the 3＇ ends of piRNAs [1, 2], however,