These cases include Harding’s disease, where the MS-like phenotype co-exists with Leber’s Hereditary Optic Neuropathy (LHON), and others where the primary mitochondrial disorder was due to a mutation in the optic atrophy 1 (OPA1) gene, mitochondrial tRNA(IIe) gene and mtDNA polymerase gamma (POLG1) gene, among others.16, , , –20 In our study, two of the three MS cases in which the respiratory enzyme-deficient muscle fibres were prominent had clinical features not usually associated with MS: Gille de la Tourette syndrome (MS04) and retinal vasculitis (MS15). Here, POLG is linked to myeloid sarcoma.