Marina, a young woman living near Varese, Italy, stood chatting with her neighbors one day two years ago when she suddenly dropped to the floor, clutching her chest. People around her panicked. But after a seemingly endless second, a violent spasm shook her body and Marina awakened. She was shocked but alive and well.

For doctors at the local hospital, the diagnosis was easy: Marina had suffered from ventricular fibrillation, a potentially lethal block of the heart muscle. After a quick checkup, she returned to her normal life. But this was no miracle: Marina survived because she was aware of a defect in her heart and took the necessary precautions.

Indeed, years before, two of her sisters had died from a rare inherited disease called stress-induced polymorphic ventricular tachycardia (VTSIP). People with this disease are at risk of fibrillation at any moment but especially after physical or emotional stress. Standard exams, such as an electrocardiogram, are usually unable to reveal the condition, but a genetic test can.

"VTSIP is caused by mutations in a gene called Ryr-2 that regulates the electric activity of the heart," says Silvia Priori, a cardiologist at the Maugeri Foundation in Pavia, Italy, who discovered the genetic alteration together with colleagues about three years ago. When doctors performed the gene test on Marina¿s DNA, they discovered that she carried the defective form of the gene. "Her risk of a having a heart attack was high," Priori says--"about 70 percent" during her lifetime.

So before any life-threatening symptoms could appear, Marina and another sister, who also tested positive, had miniature automatic defibrillators implanted in their chests. When Marina's heart fibrillated in front of her neighbors, the lifesaving device was able to jump-start the organ. An ambulance, and a regular defibrillator, would have probably arrived too late.

Know Your Genes, Save Your Life

Marina¿s story is an extreme example of a gene test that made a difference between life and death. But like Marina, hundreds of people around the world take advantage of genetic testing to detect and alter the course of various disorders. For example, hereditary hemochromatosis, or "iron disease," causes a slow accumulation of iron in many organs. Among Caucasians, it affects about 1 in 300 people, resulting in liver damage, diabetes and impotence after their 40s or 50s. Genetic testing can spot the disease in children or young adults before symptoms appear, and regular bloodletting can then prevent the condition's consequences.

The use of predictive gene tests is still limited to a handful of relatively rare and highly hereditary diseases. But that scenario is about to change: scientists in academic and corporate laboratories are tirelessly digging through human DNA to find genetic variations that make individuals susceptible to common diseases, including Alzheimer¿s, cancer, cardiovascular diseases, diabetes and stroke. Whereas today doctors calculate our risk profile for disease using a few one-size-fits-all parameters, such as cholesterol levels, blood pressure and the number of cigarettes we smoke, tomorrow they might instead consider our complex, and personalized, genetic risk pattern.

French Nobel laureate Jean Dausset was first to see the predictive value of genetic testing. "[...]medicine was, in its history, first of all curative, then preventive and finally predictive, whereas today the order is reversed: initially predictive, then preventive and finally, only in desperation, curative," he wrote. In the 1960s Dausset discovered the HLA antigens, molecules on the surface of white blood cells that are necessary for the immune response. The same antigens determine whether a transplanted organ will be rejected, which explains why it is often difficult to find a match: there are many genetically determined HLA "flavors."