Heart disease is a leading cause of mortality. The underlying pathology is typically loss of heart muscle cells that leads to heart failure. Because heart muscle has little or no regenerative capacity after birth, current therapeutic approaches are limited for the over 5 million Americans who suffer from heart failure. Our recent findings regarding direct reprogramming of a type of structural cell of the heart, called fibroblasts, into cardiac muscle-like cells using just three genes offers a novel approach to achieving cardiac regeneration. 50% of cells in the human heart are cardiac fibroblasts, providing a potential source of new heart muscle cells for regenerative therapy. We simulated a heart attack in mice by blocking the coronary artery, and have been able to reprogram existing mouse cardiac fibroblasts in to new muscle by delivering the three genes into the heart. We found a significant reduction in scar size and an improvement in cardiac function that persists after injury. The reprogramming of cells in the intact organ was more complete than in cells in a dish. We now propose to develop the optimal gene therapy approach to introduce cardiac reprogramming genes into the heart, to establish the optimal delivery approach to administer virus encoding cardiac reprogramming factors that results in improvement in cardiac function in a pig model of cardiac injury, and to establish the safety profile of in vivo cardiac reprogramming in a pig model.