Researchers are racing to complete a map on the human genome. Below, a computer-generated image of the DNA double helix, which started it all

uppose you are at risk of a genetic disease that threatens to render you unrecognizable to yourself. Physically, you realize, the disease will slowly erode your body to an incontinent mass of uncontrollable jerks and twitches. Mentally, it will eat away at your brain cells, impeding your ability to remember, pay attention, reason. And emotionally, it will blacken your days with irritability and all-consuming depression. Worst of all, you know that the disease cannot be prevented or stalled, arrested or cured. To learn that you have the offending gene is to receive a virtual death sentence that leaves only two questions unanswered: When will the nightmare begin? And how long will your suffering last?

Do you really want to know if you have the disease? Are you sure you want to know now, if the symptoms may not appear for decades? Are you prepared to handle the potential consequences of that knowledge?

Such thorny human concerns are at the heart of a pioneering research effort that is bent on clinically identifying the long-term emotional and social effects of early genetic testing. Directed by neuropsychologist Jason Brandt of the Johns Hopkins University School of Medicine, the project enlists the talents of psychiatrists, psychologists, social workers, nurses, geneticists and ethicists to track the consequences of testing for the genetic mutation that causes deadly Huntington's disease. The program, says Brandt, "is seeking to determine how best to offer this test."

Though HD afflicts just 25,000 Americans, with 125,000 more at risk, it illustrates the growing urgency to develop sound genetic-testing practices. As medical researchers race toward completing a map of the human genome, with its estimated 50,000 to 100,000 genes, they are discovering new genes, their role in specific diseases, and new diagnostic tests--all at a breathtaking pace. Within 30 years, researchers expect to be able to produce a genetic "fingerprint" of an individual's potential future health that will enable doctors to wage pre-emptive battle. Already, testing before any symptoms appear makes possible the early treatment of some breast and colon cancers. Further down the line is the prospect of gene therapy, in which modified genes are introduced into existing cells to prevent or cure numerous diseases.