Piwi-interacting RNAs (piRNAs) are small noncoding RNAs generated by a conserved pathway. Their most widely studied function involves restricting transposable elements, which are DNA elelments that jump from place to place. Another set of piRNAs now appears to have a role in the regulation of mRNA from early embryos and gonads. We report a more widespread expression of a limited set of piRNAs and particularly focus on their expression in the hippocampus. Our experiments suggested a role in spine morphogenesis, thought to be important in memory and learning. Possible targets include genes which control spine shape.