Mutations in nuclear or mitochondrial DNA may cause disorders of neuro-ophthalmic significance. These include disorders of the optic nerve, such as Leber's hereditary optic neuropathy and Kjer-type optic atrophy, and disorders of ocular motility, such as congenital nystagmus, autosomal dominant progressive external ophthalmoplegia, and oculopharyngeal muscular dystrophy. In addition to more accurate disease classification and diagnosis, identification of genetic loci, genes, and their mutations has stimulated investigation into factors influencing disease expression and penetrance.