In germ cells, early embryos, and stem cells of animals, PIWI-interacting RNAs 
(piRNAs) have an important role in silencing retrotransposons, which are vicious 
genomic parasites, through transcriptional and post-transcriptional mechanisms. 
To examine whether the piRNA pathway can be used to silence genes of interest in 
germ cells, we have generated knock-in mice in which a foreign DNA fragment was 
inserted into a region generating pachytene piRNAs. The knock-in sequence was 
transcribed, and the resulting RNA was processed to yield piRNAs in postnatal 
testes. When reporter genes possessing a sequence complementary to portions of 
the knock-in sequence were introduced, they were greatly repressed after the 
time of pachytene piRNA generation. This repression mainly occurred at the 
post-transcriptional level, as degradation of the reporter RNAs was accelerated. 
Our results show that the piRNA pathway can be used as a tool for 
sequence-specific gene silencing in germ cells and support the idea that the 
piRNA generating regions serve as traps for retrotransposons, enabling the host 
cell to generate piRNAs against active retrotransposons.