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Double-stranded RNAs can suppress expression of homologous genes through an evolutionarily conserved process named RNA interference (RNAi) or post-transcriptional gene silencing (PTGS). One mechanism underlying silencing is degradation of target mRNAs by an RNP complex, which contains ∼22 nt of siRNAs as guides to substrate selection. A bidentate nuclease called Dicer has been implicated as the protein responsible for siRNA production. Here we characterize theCaenorhabditis elegans ortholog of Dicer (K12H4.8;dcr-1) in vivo and in vitro. dcr-1 mutants show a defect in RNAi. Furthermore, a combination of phenotypic abnormalities and RNA analysis suggests a role for dcr-1 in a regulatory pathway comprised of small temporal RNA (let-7) and its target (e.g., lin-41).