Biologists hope the technique will enable replacement tissues to be generated from a patient’s own cells for use against a wide variety of degenerative diseases. For the moment, that remains a distant prospect. But the cells have already proved useful in studying the genesis of disease. Cells generated from a patient are driven to form the tissue that is diseased, enabling biologists in some cases to track the steps by which the disease is developed.

Dr. Gurdon’s early academic career did not hint at what the future might hold. “I believe Gurdon has ideas about becoming a scientist; on his present showing this is quite ridiculous,” his high school biology teacher wrote. “If he can’t learn simple biological facts he would have no chance of doing the work of a specialist, and it would be a sheer waste of time, both on his part and of those who would have to teach him.”

At Oxford University, a more positive mentor encouraged him to try transplanting the nucleus of adult cells into frog eggs. The idea was to see if the genome — the hereditary information — stayed unchanged during development or underwent irreversible changes. In producing living tadpoles from the nucleus of adult frog cells, Dr. Gurdon showed that the genome of both egg and adult cells remained essentially unchanged.

But the possibility that animals, including humans, could be cloned did not seriously impinge on the public imagination until his work was reproduced in mammals with the generation of Dolly, the cloned sheep, in 1997. The following year saw generation of the first human embryonic stem cells, which are derived from the early human embryo. Such cells are called pluripotent because they can develop into any of the mature tissues of the body.

The two developments led to the concept of therapeutic cloning — take a patient’s skin cell, say, insert it into an unfertilized human egg so as to reprogram it back to pluripotent state, and then develop embryonic stem cells for conversion into the tissue or organ that the patient needed to have replaced. Since the new tissue would carry the patient’s own genome, there should be no problem of immune rejection.

But human eggs are not so easily obtained. Of course, the reprogramming might be accomplished without human eggs if only the relevant factors in the egg could be isolated. But that seemed a distant prospect until Dr. Yamanaka’s discovery that 24 transcription factors, later whittled down to four, could reprogram a nucleus when introduced into cells on the back of a virus.