The involvement of the gene, known as AIP, in pituitary tumors is a surprise, researchers say. Mutations in the gene are associated with about 20 percent of inherited pituitary tumors when no other organ is involved. But it is not clear why mutations in this gene, which seems to be involved in metabolism — possibly to detoxify chemicals — can cause tumors or how these tumors form.

“There is nothing solid scientifically,” said Dr. Constantine Stratakis, a geneticist and pituitary tumor researcher who is the acting scientific director for the Division of Intramural Research at the National Institute of Child Health and Human Development.

And for unknown reasons, only about 30 percent of people with the mutated gene develop tumors.

In London, Dr. Korbonits said she had been aware of the Irish Giant because of her work on pituitary tumors. She suspected he might have had the AIP mutation when she saw a drawing of him standing with twin brothers who also were giants, who came from a nearby village, and who were said to be related to Mr. Byrne. That, she said, “suggested it was a genetic disease.”

And she had found the gene in members of four families from the same region of Ireland.

Dr. Korbonits wrote to the Hunterian Museum, where Mr. Byrne’s skeleton is still displayed, and asked to test the giant’s DNA, and then she and her colleagues removed two of his molars. She enlisted the help of an expert on ancient DNA, Joachim Burger of Gutenberg University in Mainz, Germany, to extract DNA from the giant’s teeth. She was worried that the DNA might be too degraded to analyze — after all, the giant’s corpse had been boiled in acid and then displayed in a museum for a couple of centuries.

“It was not clear at all that we would have suitable DNA,” Dr. Korbonits said. The DNA turned out to be broken in many pieces, but it could still be analyzed.

The investigators calculated that the giant and the four contemporary Irish families had a common ancestor who lived about 1,500 years ago. And, they report, there are probably 200 to 300 people living today who have inherited that same mutation.

One is Brendan Holland, a 58-year-old Irishman who sells mining equipment. Mr. Holland started growing excessively when he was 13, he said in a telephone interview.