In the late stage of spermatogenesis (i.e.,spermiogensis),chromatin becomes tightly packed by the replacement ofhistones to protamines via transition proteins.This process is essential for the capitulation of fertility and brings about not only dynamic chromatin changes but also global transcription suppression.Post-transcriptional regulation iscritical for this process and involves the PIWI family of proteins.The PIWI family is a conserved gene family essential for germ cell development in many organisms ranging from Drosophila to humans.PIWI family proteins play crucial roles in the production of PIWI-interacting RNAs (piRNAs),a germ cell-specific subset of small RNAs that contribute to transcriptional and post-transcriptional gene suppression,presumably through the function ofpiRNAs.1 Among three mouse PIWI family members (MIWI,MILI and MIWI2),MILI and MIWI2 have been reported to play essential roles in the gene silencing of retrotransposons by inducing de novo DNA methylation,presumably via piRNAs in embryonic testes.