Mutations in MT-CO3 gene encoding for Cytochrome c oxidase III/complex IV—respiratory chain complex IV subunit; MT-T1 gene associated with Isoleucine transfer RNA; and MT-ND4 and MT-ATP6 genes encoding for Complex V, ATP synthase, and subunit ATPase 6—respiratory chain complex V subunit are shown to impart mental retardation, cerebellar ataxias, loss of hearing, chronic progressive external ophthalmoplegia, and neuropathy in few rare HSPs [106]. The gene discussed is MCAT; the disease is External ophthalmoplegia.