Each year, one or two seemingly healthy young people die suddenly during a sports event, usually basketball. The culprit is familial hypertrophic cardiomyopathy, an inherited overgrowth of the heart muscle. The defect in this disorder is different than that behind Marfan syndrome. It is an abnormality in one of the myosin chains that comprise cardiac muscle. Again, detecting the responsible gene can alert affected individuals to their increased risk of sudden death. They can adjust the type of exercise they do to avoid stressing the cardiovascular system.