Recent years have seen a flood of studies linking genetic variations to particular diseases, and companies are trying to parlay those discoveries into direct-to-consumer genetic tests. ­Navigenics mails its subscribers containers for saliva samples. When it gets the samples back, it uses microarrays--chips studded with fragments of DNA--to screen the subscribers' DNA for genetic variations linked to 18 diseases, including Alzheimer's and colon cancer. Such genetic screening has received little clinical evaluation, however, so whether it helps prevent disease is unclear.

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