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RNA silencing. Computer artwork showing a length of RNA (ribonucleic acid, yellow with red rings) bound to an RNA-induced silencing complex (RISC). RNA is the intermediate molecule between DNA (deoxyribonucleic acid) and its protein products. Some viruses have RNA genomes as opposed to DNA genomes. RNA silencing is the degradation of targeted RNA molecules to regulate gene expression or as an antiviral defence. Double-stranded molecules of the targeted RNA are processed into short single-stranded molecules called small interfering (si) RNAs. These siRNAs are incorporated into RISC molecules and recognise and bind to complementary RNA molecules, which are then cleaved to make them inactive. (KEYSTONE/SCIENCE PHOTO LIBRARY/MEDI-MATION)