Explaining his work in immunology, Tonegawa gave reporters a sense of what he is like as a professor.
He used an analogy involving General Motors to explain his pioneering research on how the body can use limted amounts of genetic material to produce millions of antibodies to defend itself against disease:
"When General Motors builds a car, they want to meet the specific needs of many customers. But if they custom-make each car, then it will not be economical.
"So what they do is they make all kinds of parts - different forms and shapes and function and color and so on - and they assemble them in different ways. Therefore, one can make all kinds of different cars.
"So, in a way, one could look at that. It's a matter of how you assemble these pieces (of genetic material) to generate the diversity (of antibodies)."
Before Tonegawa's ground-breaking research in 1976, some scientists thought that each person was born with all of the specific genetic information necessary to produce every possible antibody needed to counter the thousands of foreign molecules that might invade the body with bacteria and viruses, for example.
"This was one of the most hotly debated issues in immunology," Tonegawa said. "We did not understand how that happens."
In a study published in 1976, he compared genetic material from two sources: the immune-system cells of adult mice, and the cells from mouse embryos. He found evidence that genes had rearranged themselves into different combinations to produce the necessary variation of antibodies, he said.
"We found out that, contrary to what many people thought, in the immune system genes can change during the life cycle of the individual. That finding was unexpected, I guess . . . and . . . answered one of the mysteries of immunology," he said.
Tonegawa's colleagues describe him as a soft-spoken man who is intensely dedicated to his work.
Andre Augustin, a researcher from the University of Colorado who has known Tonegawa since they worked together in Switzerland in 1974, said, "He is a person who works very hard. I would say that opposed to many other people who use a lot of people to work for him, he does much of his work himself."
Aside from his work, Tonegawa, 48, is a fledgling collector of paintings and an avid Boston Red Sox fan, Augustin said.
Last year, when the Sox lost the World Series to the New York Mets, "he suffered a lot," said Augustin. "He knows every player and who is married and who is single. He thinks that football is too aggressive. He thinks that baseball refects the individual players."
Tonegawa, who is still a Japanese citizen, said it was not important to him that he was the first from his nation to win the Nobel prize in medicine.
"I've been telling people, particularly young Japanese scientists, that science should be international and one should not be affected by the fact that you are born in a particular country with a particular culture," he said.
He said he never did research in Japan and felt more comfortable working in the United States.