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Complex III is located within the inner membrane of the mitochondria and is the second enzyme in the electron transport chain of the oxidative phosphorylation process. The ubiquinol cytochrome c reductase catalyses the electron transfer from succinate and nicotinamide adenine dinucleotide-linked dehydrogenases to cytochrome c within the respiratory chain located within the inner membrane of the mitochondria. The clinical features usually include progressive ataxia, predominantly proximal muscle weakness, areflexia, extensor plantar responses, dementia, and concomitant nonspecific myopathic and neuropathic changes in muscle. External ophthalmoplegia, ptosis and cardiomyopathy are often present. The most frequent clinical condition is Leber Myopathy.

Ubiquinone-cytochrome Oxidoreductase; Coenzyme Q-Cytochrome C Reductase Deficiency.

The incidence for Complex III deficiency remains unknown.

Inheritance is usually autosomal recessive, however, an autosomal dominant inheritance has also been suggested. Mutations in the BCS1L gene on chromosome 2q33 or the UQCRB gene on chromosome 8 have been demonstrated. The BCS1L gene are associated with tubulopathy, encephalopathy, and liver failure. GRACILE syndrome, which belongs to the Finnish disease heritage, is also caused by mutation in the BCS1L gene, but displays a different phenotype.

Seijo-Martinez M, Castro del Rio M, Campos Y, et al: Unusual clinical findings and Complex III deficiency in a family with myotonic dystrophy. J Neurol Sci 208:87, 2003.  [PubMed: 12639730]
Thyagarajan D, Byrne E; Mitochondrial disorders of the nervous system: Clinical, biochemical, and molecular genetic features. Int Rev Neurobiol 53:93, 2002.  [PubMed: 12512338]

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