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Harding's disease: an important MS mimic. Leber's hereditary optic neuropathy (LHON) is a mitochondrially inherited disorder characterised by bilateral, painless visual loss which leads to severe optic atrophy. It can be associated with other conditions including multiple sclerosis (MS), movement disorders, epilepsy and cardiac arrhythmias. The association of LHON with an MS-like illness is often referred to as Harding's disease (or Harding's syndrome). We report two siblings, who both harbour the 11 778 mitochondrial DNA (mtDNA) mutation, but who manifest markedly different clinical phenotypes; a male with classical LHON and a female with an MS-like illness. LHON affects males four to five times more often than females. By contrast, Harding's disease is seen predominantly in females, in a pattern comparable to that seen in MS. The pathogenic basis behind the variation in penetrance and phenotype between genders and individual family members remains unclear.