Progressive external ophthalmoplegia ( is characterized by multiple mitochondrial DNA deletions in skeletal muscle. The most common clinical features include adult onset of weakness of the external eye muscles and exercise intolerance. Additional symptoms are variable, and may include cataracts, hearing loss, sensory axonal neuropathy, ataxia, depression, hypogonadism, and parkinsonism. Both autosomal dominant and autosomal recessive inheritance can occur; autosomal recessive inheritance is usually more severe (Filosto et al., 2003Luoma et al., 2004).

PEO caused by mutation in the POLG gene is associated with more complicated phenotypes than those forms caused by mutation in the ANT1 or C10ORF2 genes (Lamantea et al., 2002).