POLG and Leber hereditary optic neuropathy: Such cases have been reported in Harding's disease, where MS-like disease co-exists with Leber's hereditary optic neuropathy (LHON), and others where the primary mitochondrial disorder was identified as a mutation in the optic atrophy 1 (OPA1) gene, or the mitochondrial DNA polymerase gamma (POLG1) gene.