Within 1 year large-scale deletions in the mtDNA causing mitochondrial myopathy (MIM 251900) or Kearns-Sayre syndrome (KSS, MIM 530000) shortly followed by an mtDNA point mutation in MT-ND4 causing LHON (MIM 5350000), were identified (Holt et al., 1988; Wallace et al., 1988; Zeviani et al., 1988). Here, MT-ND4 is linked to Kearns-Sayre syndrome.