Mitochondrial myopathy in a child with a muscle-restricted mutation in the mitochondrial transfer RNAAsn gene

► Exercise intolerance is associated with mutations in mitochondrial DNA genes. ► A 11-year-old boy with exercise-related myopathy showed a novel mt-tRNAAsn mutation. ► Extensive investigations supported the pathogenicity of this mutation. ► A mitochondrial myopathy should be considered in children with exercise intolerance.