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Double-stranded RNA can induce the degradation of homologous RNAs in organisms as diverse as protozoa, animals, plants and fungi, resulting in post-transcriptional gene silencing. But in some species, RNA-mediated processes can also lead to translational repression, DNA methylation, heterochromatin formation or DNA elimination. In some situations, amplification of the “trigger” double-stranded RNA seems to be required for efficient silencing. These findings imply that RNA-mediated mechanisms can control gene expression at both the transcriptional and the post-transcriptional level, and that they can operate in the nuclear and the cytoplasmic compartments.