For example, mutations in AFG3L2 lead to spinocerebellar ataxia type 28 [49], or spastic ataxia 5 [50]; mutations in paraplegin are the causative agents of hereditary spastic paraplegia [51] and progressive external ophthalmoplegia [52]; and mutations in both AFG3L2 and SPG7 occur in dominant optic atrophy (DOA) [53]. Here, SPG7 is linked to progressive external ophthalmoplegia.