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.