The 2006 Nobel Prize for Medicine will be shared by Stanford's Andrew Fire and the University of Massachusetts' Craig Mello for their detective work on the phenomenon of RNA interference (RNAi), which has the potential to revolutionise medicine.

RNAi enables geneticists to design molecules to "silence" specific genes. It's early days for the field, but the target could be a malfunctioning copy of one of our own genes, as in many cancers where cells are reproducing out of control. The faulty gene responsible for sickle cell anaemia, one of the world's most commonly inherited diseases, could be knocked out by an RNAi molecule.

Alternatively, genes key to a multiplying bacterium or virus, like HIV, could be knocked out and the infection halted. Trials in mice have already been successful. The never-ending battle with viruses is thought to have been the driver for the emergence of RNAi early in the evolution of multicellular life - the apparatus is shared by plants and animals.

The value of RNAi to basic science is enormous too. It makes the herculean task of identifying the function and interaction of each of the genes discovered by the Human Genome Project easier.

It was first observed as part of a plant breeding programme in the late 1980s, where genetically engineered petunias did not develop the colours they were expected to. Instead of deep purples and reds, the flowers were white or part-white.

The molecular mechanism of how the petunias silenced the pigment genes which had been inserted into them remained obscure. Fire and Mello worked out that specially targeted RNA molecules were responsible for the knockout.

RNA - RiboNucleic Acid - is a close chemical relative of DNA. It acts as the genetic workhorse in cells: if DNA is an encyclopaedia, RNA is a daily newspaper; shorter, quicker, throwaway, but powerful nonetheless.

The Nobel Foundation is announcing the 2006 recipient of one of its prestigious gongs every day this week, climaxing with the Peace Prize on Friday.

Fire and Mello will share a purse of 10 million Swedish Kronor (about £730,000). The worth of their discoveries to the biotech and medical industries will likely dwarf that figure. More importantly, the value of RNAi to disease sufferers will not be measured in currency. ®

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