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In plants and fungi, the introduction of transgenes can lead to post-transcriptional gene silencing,. This phenomenon, in which expression of the transgene and of endogenous genes containing sequences homologous to the transgene can be blocked, is involved in virus resistance^,, and genome maintenance,. Transgene-induced gene silencing has been termed quelling in Neurospora crassa and co-suppression in plants. Quelling-defective (qde) mutants of N. crassa, in which transgene-induced gene silencing is impaired, have been isolated. Here we report the cloning of qde-1, the first cellular component of the gene-silencing mechanism to be isolated, which defines a new gene family conserved among different species including plants, animals and fungi. The qde-1 gene product is similar to an RNA-dependent RNA polymerase found in the tomato. The identification of qde-1 strongly supports models that …