cytochrome b mutations can frequently cause isolated exercise intolerance and myopathy and in some cases multisystem disorders. the mitochondrial respiratory chain complex iii catalyses electron transfer to cytochrome c. complex iii is embedded in the inner membrane of the mitochondria and consists of 11 subunits. cytochrome b is encoded by the mitochondrial dna which differs from all other subunits which are encoded in the nucleus. cytochrome b plays a major part in the correct fabricating and function of complex iii.