Belongs to the heterogeneous family of metabolic myopathies. It is characterized by progressive exercise intolerance manifesting in childhood, onset of sideroblastic anemia around adolescence, lactic acidemia and mitochondrial myopathy. Less than 10 cases have been described so far. A 656C-->T mutation in the nuclear pseudouridine synthase 1 gene (PUS1), localized to 12q24.33, has recently been identified in some patients. Deficient pseudouridylation of mitochondrial tRNAs may be responsible for the oxidative phosphorylation disorder. Transmission is autosomal recessive.