Ready to dice In the biogenesis of small RNAs, the Dicer family endonucleases act as a molecular ruler to cut the substrate RNA into defined lengths. Wang et al . report a structure of the dicing-competent state of plant DICER LIKE PROTEIN 3 (DCL3) in complex with a pre–small interfering RNA (pre-siRNA). Toward one end of the pre-siRNA, DCL3 uses a positively charged pocket and an aromatic cap to specifically recognize the 5′-phosphorylated adenosine of the guide strand and the 3′ overhang of the complementary strand, respectively. On the other end, the paired ribonuclease III domains of DCL3 cut both strands of the RNA, determining the precise length of the product small RNA. —DJ