Exercise intolerance due to cytochrome b mutation

Cytochrome b mutations are rare causes of exercise intolerance. We report an 18‐year‐old man with exercise intolerance since childhood, resting lactic acidosis, cytochrome c oxidase (COX)‐positive ragged‐red fibers, and isolated muscle complex III deficiency due to a heteroplasmic m.14849T>C mutation in cytochrome b . We review previously described patients carrying mutations in the same gene. COX‐positive ragged‐red fibers together with exercise intolerance and lactic acidemia provide a clue for the diagnosis of this rare mitochondrial disorder. Muscle Nerve, 2010