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A rare mitochondrial oxidative phosphorylation disorder characterized by a highly variable phenotype which may present as exercise intolerance with prominent exertional dyspnea, progressive muscle weakness, spasticity, and neuropathy, but without cognitive impairment or cardiac involvement, or as global developmental delay, growth retardation, hypotonia, and spasticity. Hypertrophic cardiomyopathy, optic atrophy, seizures, and dysmorphic facial features have also been reported in the more severe phenotype. Serum lactate may be elevated, and muscle biopsy shows myopathic features and variably decreased activity of mitochondrial respiratory chain complexes.