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Jeffrey Gulcher had no reason to think much about prostate cancer. He was just 48, and the disease typically strikes later in life. Even the most cautious medical groups agree that most men need not begin annual prostate screenings until age 50.

But Gulcher happens to be the chief scientific officer of deCODE Genetics -- one of several companies that, amid some controversy, have begun offering direct-to-consumer DNA tests that can help people predict which diseases they are likely to get. So in April, he spat into a test tube and, without giving the matter much thought, sent the sample in for analysis by his own company.

He was in for a shock. The test indicated that he carries a genetic variant that nearly doubles his lifetime risk of getting prostate cancer: While the average man has a 15 percent chance of being stricken, Gulcher had a 30 percent shot. That spurred his physician to order a standard blood test for prostate cancer. The result was toward the high end of the range considered normal, which, together with the DNA...