STOCKHOLM — Americans Mario R. Capecchi and Oliver Smithies and Briton Sir Martin J. Evans won the 2007 Nobel Prize in medicine today for groundbreaking discoveries that led to a technique for manipulating mouse genes.

The widely used process has helped scientists use mice to study heart disease, diabetes, cancer, cystic fibrosis, and other diseases.

Mr. Capecchi, 70, who was born in Italy, is at the University of Utah in Salt Lake City. Mr. Smithies, 82, born in Britain, is at the University of North Carolina in Chapel Hill. Mr. Evans is at Cardiff University in England.

They were honored for a technique called gene targeting, which lets scientists inactivate or modify particular genes in mice. That in turn lets them study how those genes affect health and disease.

The first mice with genes manipulated in this way were announced in 1989. More than 10,000 different genes in mice have been studied in this way, the Nobel committee said. That’s about half the genes the rodents have.

“Gene targeting has pervaded all fields of biomedicine. Its impact on the understanding of gene function and its benefits to mankind will continue to increase over many years to come,” the award citation said.

Mr. Capecchi’s work has uncovered the roles of genes involved in organ development in mammals, the committee said. Mr. Evans has developed strains of gene-altered mice to study cystic fibrosis, and Mr. Smithies has created strains to study such conditions as high blood pressure and heart disease.

The medicine prize was the first of the six prestigious awards to be announced this year. The others are chemistry, physics, literature, peace, and economics.