FGF21 and mitochondrial disease: We are currently extending this study to include more patients with mitochondrial diseases (with and without myopathy), non-mitochondrial myoapthies and various neurometabolic disorders, to correlate GDF-15 plasma levels with other biochemical markers (in particular FGF-21, lactate, pyruvate, lactate to pyruvate ratio and creatine kinase values) [52] as well as with muscle histopathological markers such as COX negative and/or ragged-red fibres.