Mitochondrial diseases are whole body diseases but the exact features of the disease vary from one individual to another. Some patients will have predominantly brain disease or nerve disease. Others will have muscle disease (mitochondrial myopathies), cardiac disease (cardiomyopathies), endocrine, renal or bone marrow disease or a mixture of these and other features. Many mitochondrial diseases result in the accumulation of organic acids in the body. These are usually normal metabolic intermediates but when present in excess the acidosis itself may he damaging or even life threatening. Lactic acid accumulation is a common problem in mitochondrial diseases.