Mitochondrial disease is a group of genetic conditions that affect how mitochondria in your cells produce energy. Mitochondria are responsible for producing energy within your body, and when they don’t receive the instructions they need from your body’s DNA to make energy, it can damage your cells or cause them to die early. This affects how your organs and organ systems function, which leads to symptoms of the condition. Mitochondrial diseases can affect almost any part of your body, including the cells of your heart, brain, muscles, nerves, eyes, ears, pancreas, kidneys, and liver. The parts of your body that need the most energy, such as the heart, brain, and muscles, are most affected by mitochondrial disease. Symptoms of mitochondrial disease can vary widely, depending on which cells of the body are affected. There is no cure for mitochondrial disease, but treatment can prevent life-threatening complications.