A nonsense mutation (G15059A) in the cytochrome b gene in a patient with exercise intolerance and myoglobinuria

We describe a new mitochondrial DNA mutation in the cytochrome b gene in a patient presenting with progressive exercise intolerance and myoglobinuria associated with complex iii deficiency in muscle. The point mutation result s in the replacement of a glycine at amino acid position 190 with a stop co don. This change predicts premature termination of translation, leading to a truncated protein missing 244 amino acids at the C-terminus of cytochrome b. The mutation fulfills all the accepted criteria for pathogenicity, sugg esting that this is the primary cause of the myopathy in the patient.