Two Americans and a Briton were awarded the 2007 Nobel Prize in physiology or medicine yesterday for their work in creating "designer mice" - experimental animals in which genes have been added or removed to test theories about the links between genes and disease.

Along with former Bucks County resident Mario Capecchi, 70, of the University of Utah; Oliver Smithies, 82, of the University of North Carolina at Chapel Hill and Martin Evans, 66, of Cardiff University in Wales will share the $1.54 million prize for work that the Nobel committee said "has revolutionized life science and plays a key role in the development of medical therapy."

Evans isolated embryonic stem cells from mouse tumors, providing a tool that could be used to introduce new genes into mouse strains, and is widely considered the father of embryonic-stem-cell research. Such cells have the ability to turn into any other type of cells in the body, and researchers hope they will eventually be useful in treating a host of diseases.

Capecchi and Smithies independently developed techniques to target individual genes within an organism and eliminate or replace them with a slightly altered form that could then be passed down to descendants.

The "knockout" mice, in which specific genes have been eliminated, allow researchers to demonstrate exactly what the gene does in a living organism. Smithies compared the feat to removing the steering wheel from a car, for example, to see what role it plays in driving the vehicle.

Since the researchers reported their results in the 1980s, mouse strains have been produced in which about half the mouse genomes' 22,000 genes have been individually eliminated. Researchers expect all the mouse genes to have received the treatment within the next few years.

The knockout mice also provide animal models for diseases, allowing researchers to test new drugs and treatments.

The three received the 2001 Albert Lasker Award for Basic Medical Research, an award often assumed to be a precursor of the Nobel.

Both awards were richly deserved because their work "has dramatically reshaped the research landscape," said Jeremy Berg, director of the National Institute of General Medical Sciences, which funded most of Capecchi's and Smithies' research. It has created "an indispensable tool for biomedical research."

Smithies was born in Yorkshire, England, and educated at Oxford University before immigrating to the United States in the 1950s at the suggestion of his research supervisor, Sandy Ogsten. "I wasn't particularly keen on it," but it worked out well, he said.

He said that the financial reward was not as important as "the feeling that people appreciate the science you have done. I've spent 50 years as a bench scientist, and the first paper [on the Nobel-winning research] came after I was 60 years old. It shows that . . . one shouldn't be in a hurry to throw people out because they are old."

Evans was visiting his daughter in Cambridge when he received news of the prize. "I haven't come to terms with it yet," he said. "In many ways, it is the boyhood aspiration of science, isn't it?"

He was knighted by Queen Elizabeth on Jan. 1, 2004, on his 63d birthday "for his contributions to medical science."

The trio will receive their awards in Stockholm on Dec. 10.