In a report published online in advance in Science this week, University of Cambridge's Eric Miska and his colleagues discuss the roles of piwi-interacting RNAs in Caenorhabditis elegans. "Our data suggest that nematode piRNA clusters are evolving to generate piRNAs against active mobile elements," Miska et al. write. "Thus, piRNAs provide heritable, sequence-specific triggers for RNAi in C. elegans."