A study in human and mouse heart cells identifies a faulty molecular brake in the most common form of hypertrophic cardiomyopathy, a leading cause of sudden cardiac death in young people and athletes and the most common genetic disease of the heart. The faulty brake, found about a quarter of all genetic mutations in hypertrophic cardiomyopathy, interferes with the heart muscle’s ability to contract and relax,. Treatment with a chemical compound successfully restores normal contractility and relaxation in human heart cells.