piRNAs initiate an epigenetic memory of nonself RNA in the C. elegans germline.
Shirayama M, Seth M, Lee HC, Gu W, Ishidate T, Conte D, Mello CC. piRNAs initiate an epigenetic memory of nonself RNA in the C. elegans germline. Cell. 2012 Jul 06; 150(1):65-77.