*2.1. Subjects*

This study was approved by the Ethics Committee IRB 00001360 affiliated with Vector State Research Center of Virology and Biotechnology (SRC VB Vector), Novosibirsk, Russian Federation. The total number of subjects in the study is 168 individuals from 44 unrelated families (85 affected and 83 healthy carriers), including 17 cases from our previous studies [21–23]. The clinical follow-up of LHON patients has been carried out by the Novosibirsk Branch of Federal Eye Microsurgery Department since 1997, conducted by one of the authors. The clinical diagnosis was based on a combination of symptoms and signs: painless acute or subacute central vision loss; fundus changes; and visual field abnormality, such as pseudopapilledema, optic nerve atrophy, and central or centrocecal scotoma. All the individuals made an informed decision to take part in the study and provided written consent. Family history was taken in each case to identify maternal inheritance of symptoms. The complete mtDNA genomes were sequenced for the family's probands, and for the other individuals the certain mutations were confirmed by sequencing of associated mtDNA regions.
