A man whose genetic defect helped to further medical understanding of HIV, the virus that causes Aids, has died, his family said.
Stephen Crohn, 66, who was once called "The man who can't catch Aids," committed suicide in New York.
His sister Amy Crohn Santagata told the New York Times: "My brother saw all his friends around him dying, and he didn't die.
"He went through a tremendous amount of survivor guilt about that and said to himself 'There's got to be a reason.' He was quite extraordinary."
Mr Crohn, an artist and freelance travel editor, was the great-nephew of gastroenterologist Burrill B Crohn, who was among the first to describe Crohn's disease.
Doctors studied him and eventually established that his white blood cells had a rare defect which meant they would not allow the HIV virus in.
Dr Bruce Walker of the Ragon Institute, told the New York Times: "What he contributed to medical knowledge is really quite extraordinary.
"This is a classic case of medical science learning from patients. You take the extreme examples and try to see how those people are different from the average person with the disease."