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The reprogramming allows scientists to create particular kinds of tissue they want to study, such as lung tissue for studying cystic fibrosis, or brain tissue for Huntington’s disease. By reprogramming cells from patients with a particular disease, they can create new tissue with the same genetic background and study it in the lab. That can give new insights into the roots of the problem.

In addition, that approach allows them to screen drugs in the lab for possible new medicines.

Experts welcomed the announcement, praising the duo for their groundbreaking and influential discoveries in a field riddled with ethical debates.

“Everyone who works on developmental biology and on the understanding of disease mechanisms will applaud these excellent and clear choices for the Nobel Prizes,” said John Hardy, professor of neuroscience at University College London. “Countless labs’ work builds on the breakthroughs they have pioneered.”

Mr. Yamanaka deserves extra credit for overcoming fierce objections to the creation of embryos for research, reviving the field, said Julian Savulescu, director of Oxford University’s Uehiro Centre for Practical Ethics.

Yamanaka has taken people’s ethical concerns seriously about embryo research and modified the trajectory of research into a path that is acceptable for all,” Mr. Savulescu said. “He deserves not only a Nobel Prize for Medicine, but a Nobel Prize for Ethics.”

Goran Hansson, the secretary of the prize committee, said he had reached both winners by phone before the announcement. He said they were looking forward to coming to Stockholm to collect the 8 million kronor ($1.2 million) award on Dec. 10.

The medicine award was the first Nobel Prize to be announced this year. The physics award will be announced Tuesday, followed by chemistry on Wednesday, literature on Thursday and the Nobel Peace Prize on Friday.

The economics prize, which was not among the original awards but was established by the Swedish central bank in 1968, will be announced on Oct. 15. All prizes will be handed out on Dec. 10, the anniversary of prize founder Alfred Nobel’s death in 1896.

Last year’s medicine award to Canadian-born Ralph Steinman, American Bruce Beutler and Frenchman Jules Hoffmann briefly created some confusion when it was announced that Steinman had died a few days earlier. Posthumous prizes are normally not allowed, but the award was left unchanged since the judges were not aware of Steinman’s death when they selected him as a winner.

Associated Press science writer Malcolm Ritter in New York and writers Cassandra Vinograd in London and Elaine Kurtenbach in Tokyo contributed to this report.