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The expanding use of genetic testing is having an unforeseen consequence: More people are being told they have genes for potentially fatal diseases but don't show any symptoms.

Experts call these people patients-in-waiting. They can live in medical limbo for years while they and their doctors wonder if and when a disease, such as cystic fibrosis, will develop. Coming down with a simple cough or fever may prompt some patients to fear it is an early sign of a disorder. Another dilemma: Doctors are sharply divided about whether to begin treatment in hopes of preventing the disease's onset.

At Children's Hospital Los Angeles, the cystic-fibrosis clinic tracks 250 patients. Of these, 70 are patients-in-waiting. Danieli Salinas, a pediatric pulmonologist at the clinic, says she tells these patients and their families: "You have the genetic potential to develop cystic fibrosis, which could be next month, next year, when you are 60 years old, or never." Cystic fibrosis is a disorder caused by gene changes inherited from both parents that affects the lungs and digestive system of some 30,000 Americans.

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