Oliver Smithies Credit: AP Photo/Karen Tam

The architects of a technique that has allowed biologists to identify the function of genes easily have been rewarded for their efforts with this year's Nobel Prize in Physiology or Medicine.

The technique allows researchers to generate 'knock-out' mice -mutant strains in which specific genes are disabled. These can be used to establish what role specific genes have in health, development and disease, and to create animal models of human diseases.

"Virtually no field of biomedicine has been untouched by one knock-out strain or another in a significant way," says Jeremy Berg, director of the National Institute of General Medical Sciences in Bethesda, Maryland.

Mario Capecchi Credit: AP Photo/Douglas C. Pizac

Mario Capecchi from the University of Utah in Salt Lake City, Martin Evans of Cardiff University in Wales and Oliver Smithies of the University of North Carolina at Chapel Hill, share the ?1.1-million (US$1.5-million) award. As is now often the case with this Nobel prize, the trio's work had previously been recognized with a Lasker award, in 2001.

Thousands of strains of knock-out mice have been generated since use of the technique was first reported in 1989. More than 500 of these are models for specific human disorders such as cardiovascular and neurodegenerative diseases, and cancer.

Martin Evans

The technology's origins lie in a natural phenomenon called homologous recombination, which cells are thought to exploit to repair damaged DNA. Chromosomes, which package DNA, exist in pairs ? one inherited from each parent ? and during homologous recombination fragments of DNA can be exchanged between the two. Capecchi and Smithies found that artificial DNA of known sequence could engage in homologous recombination with mouse DNA, and exploited this to target specific mouse genes.

Evans provided the key element of heritability that eventually led to the development of knock-out mice ? strains in which a gene remains knocked out in future generations. He had the idea of using mouse embryonic stem cells to introduce genetic material into embryos from a different strain of mouse. When he injected the stem cells into the embryos, their chromosomes, as expected, combined. The mosaic embryos created could be brought to term in a surrogate mother. When the pups were mated, their offspring contained genes derived from the stem cells. Evans then began modifying the stem cells before injecting them into the mouse eggs, using retroviruses to integrate new genes into their genomes. These new genes could then be transferred into the embryos and their offspring. Combining this technique with artificial homologous recombination led to the development of the first knock-out mouse.

Important refinements to the technology ? particularly the development of 'conditional mutants' ? have made knock-outs even more valuable for biologists. A system, known as Cre-lox, has been developed in mice by Klaus Rajewsky, now of Harvard Medical School, to allow the targeted gene to be switched off at a chosen time after birth. This is important both because up to 15% of genes are essential for embryonic development and a knock-out would not survive to birth, and because some genes may become relevant for a particular disease only later in life.

Knock-out mice

Smithies told Nature that the prize was "not unexpected, given the calibre of the work". Celebrating with ice-cream, he confessed to "being pleased". What thrills him most, he says, is opening journals to find so many papers that rely on knock-out mice. "It's quite clear that the homologous-recombination method has contributed deeply to our understanding of the genome," he says. "We may have the sequence, but knock-outs help us understand how it functions."

In recent years, Capecchi has helped elaborate the role of particular genes in embryonic development. His work has focused particularly on body organs and the way the body plan is formed ? vital to ensuring that all of an animal's body parts are in the correct places.

Evans has developed many mouse models of important human diseases, such as cystic fibrosis, and has used these to study disease mechanisms and to try to find ways to repair defective genes. Smithies has also developed a cystic fibrosis mouse as well as models of common diseases including high blood pressure and atherosclerosis.

Now that the mouse genome has been sequenced, a worldwide effort has been launched to knock out every single one of the animal's genes.

"The impact of the technology on the understanding of gene function and its benefits to mankind will continue to increase for many years to come," says Evans.