Researchers have managed to restore heart function by transplanting muscle stem cells into damaged mouse hearts. Their results suggest that the technique could one day be used to heal heart tissue in humans. Similar transplants have been tried before in both mice and humans, but have met with little success. Although the grafts sometimes improved the function of the heart, they also raised the risk of abnormally fast heartbeats, in a disorder called ventricular tachycardia. Ventricular tachycardia is the main cause of sudden death in patients who have had a heart attack, killing about 15% of patients within three years...