"The data is only as good as the interpretation," says Joel Dudley, a professor at Mount Sinai School of Medicine. Most companies use similar technology to identify genetic variations. How a company translates these to tell you something meaningful about your health is the measure of its worth. A quality outfit has a staff of scientists and analysts to do just that, says University of Toronto genetics professor Ahmed El-Sohemy: "Avoid some business guy with a bachelor's degree in genetics."