Piwi-interacting RNAs (piRNAs) are small non-coding RNAs that have an unclear function. piRNA expression was thought to be limited to germline cells, but in a new study, Kandel and colleagues have detected piRNAs in a microRNA library from the CNS of Aplysia californica. Experiments involving knockdown of Piwi in cultured A. californica neurons revealed that Piwi–piRNA complexes enhance serotonin-induced long-term facilitation — a form of synaptic plasticity implicated in long-term memory. These complexes exerted their effects through methylation of the promoter region of the gene encoding CREB2, a major transcriptional repressor of memory formation in A. californica, leading to a decrease in CREB2 expression.