Thus, in the two patients with a homoplasmic mutation in the mtDNA gene MT-ATP6 (P14 and P15), the highest sNFL levels were detected in P15, with ataxia, dysarthria, epilepsy, optic atrophy, cognitive impairment and polyneuropathy as compared to P14 with polyneuropathy. The gene discussed is MT-ATP6; the disease is Leber hereditary optic neuropathy.