"RNA silencing" refers to related processes of post-trancriptional control of gene expression found in plants, animals and fungi. A unifying feature of RNA silencing is that it mediates sequence-specific degradation of target transcripts, recruiting RNA molecules of 21-23 nucleotides as specificity determinants. In higher plants, RNA silencing serves as an adaptive, antiviral defence system, which is transmitted systemically in response to localized virus challenge. Plant viruses have elaborated a variety of counter-defensive measures to overcome the host silencing response. One of these strategies is to produce proteins that target the cell autonomous or signalling steps of RNA silencing. It is not known whether a similar antiviral mechanism also operates in animal cells.