Dr. Stefansson says that in the future, people could test themselves for bad version of the gene at a relatively young age.

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DeCode's heart gene hunt was led by an unassuming young researcher named Anna Helgadottir, who spent four years scanning the genomes of thousands of Icelanders and constantly worrying that the efforts might yield nothing.

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Some 600, 000 Americans have a less severe version caused by one defective copy. (There would be more of the double-gene victims if they didn't die young.) To get its drug approved Isis will need to study 45 patients with the severe two-gene form and a few hundred with the single-gene disease.

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For the last 15 years, as prices for young horses have risen, the thoroughbred gene pool has been shrinking.

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They found a gene called SI00A4 was strongly associated with tumours in very young children.

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Sure enough, geneticists discovered that patients with rare gene mutations that cause them to produce high levels of the 42 form usually get Alzheimer's at a young age.

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