The 2006 Nobel Prize for Medicine has been awarded to the US researchers Dr Andrew Fire (Stanford University) and Dr Craig Mello (University of Massachusetts), for their discovery of RNA interference (RNAi). Published in Nature in 1998, the discovery of RNAi revolutionised understanding of the role of RNA in the regulation of gene expression. In brief, RNA interference is a gene silencing mechanism that prevents expression of transcribed genes (conversion from messenger or mRNA molecules into protein products) via double-stranded RNA molecules; the dsRNAs activates cellular machinery that targets and selectively degrades mRNAs identical to the dsRNA sequence. Targeting specific mRNAs for destruction prevents expression of the corresponding gene.

Animal and plant genomes contain multiple microRNAs (miRNAs) that encode small portions of coding sequence, and which allow the formation of double-stranded RNAs. It is now known that this form of genetic regulation via miRNAs is important in normal cellular processes, and in the growth and development of organisms. RNA interference is also an important basic cellular defence mechanism against viruses (many of which have double-stranded RNA genomes) and mobile genetic elements such as transposons, which move via processes including the formation of double-stranded RNA intermediates and can disrupt important genomic sequences.

The Nobel citation issued by the Swedish Karolinska Institute stated: "This year's Nobel Laureates have discovered a fundamental mechanism for controlling the flow of genetic information" (see press release). The discovery has already lead to a wide range of research applications, using specifically designed dsRNA molecules to silence specific genes; there are also prospects for harnessing the process as a therapeutic tool to treat genetic disorders, viral infections and a range of other diseases.