OPA3 mutations were first identified in Iraqi Jewish families with Type III 3-methylglutaconic aciduria (Costeff syndrome); an autosomal recessive neurodegenerative disorder characterised by optic atrophy, progressive neurodegeneration, increased urinary levels of 3-methylglutaconic acid, and elevated plasma 3-methylglutaric acid levels (Costeff et al., 1989; Anikster et al., 2001). Here, OPA3 is linked to hereditary optic atrophy.