Since the symptoms of mitochondrial disorders and metabolic myopathies are very unspecific, they can easily be mistaken for i.e. cardiopulmonary disease and diagnosis can be difficult. Therefore, it would be useful to have a biomarker that could easily distinguish both disorders from others. A recent study showed that Growth Differentiation Factor 15 (GDF-15) was significantly elevated in blood from patients affected by mitochondrial disorders as compared to healthy individuals, but it is unknown whether this increase is specific for mitochondrial disease. Thus it is unknown how GDF-15 levels are in patients with other muscle disease, including metabolic myopathies in which an energy deficiency, as in mitochondrial diseases, is also present. GDF-15 belongs to the transforming growth factor beta super family of growth factors that regulates inflammation and apoptosis in injured tissue. It is not known why GDF-15 is elevated, but it has been suggested that the oxidative stress, which is a part of the pathophysiology in mitochondrial disorders, increases GDF-15 through activation of P53. Oxidative stress also plays a role in the pathophysiology of some metabolic myopathies [8], and GDF-15 may therefore be elevated in these patients too. To make sure that the GDF-15 is not elevated due to muscle involvement, it will also be measured in a subgroup of muscular dystrophies.