“Like any scientist, I like solving puzzles,” he said in an interview this morning.

But he had an unprepossessing start. When he was a pre-med student hoping to become a physician researcher, a professor wrote, “Mr. Kaelin appears to be a bright young man whose future lies outside of the laboratory.”

Eventually he became intrigued by a rare, genetic cancer, von Hippel-Lindau disease, that is characterized by a profusion of extra blood vessels and overproduction of erythropoietin, or EPO, a hormone that stimulates production of the red blood cells that carry oxygen.

The cancer “was really fascinating,” Dr. Kaelin said. It had unusual features, like causing the body to make a substance, vegF, that stimulates the formation of blood vessels. And the cancer can cause the body to make too many red blood cells by increasing the production of EPO.

He had a hunch about what was going awry: “I thought it had something to do with oxygen sensing.”

As it turned out, he was right.

“It is one of the great stories of biomedical science,” Dr. Daley said. “Bill is the consummate physician-scientist. He took a clinical problem and through incredibly rigorous science figured it out.”

Dr. Kaelin said he knew, of course, that today the Nobel Prize would be awarded. But his chances were “so astronomically small” that he stuck with this usual routine and did not stay up last night.