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Objectives: Leigh syndrome (LS) is one of the most common mitochondrial diseases and has variable clinical symptoms. However, the genetic variant spectrum of this disease is incomplete. Methods: Next-generation sequencing (NGS) was used to identify the m.14430A > G (p.W82R) variant in a patient with LS. The pathogenesis of this novel complex I (CI) variant was verified by determining the mitochondrial respiration, assembly of CI, ATP, MMP and lactate production, and cell growth rate in cybrids with and without this variant. Results: A novel m.14430A > G (p.W82R) variant in the NADH dehydrogenase 6 (ND6) gene was identified in the patient; the mutant loads of m.14430A > G (p.W82R) in the patient were much higher than those in his mother. Although the transmitochondrial cybrid-based study showed that mitochondrial CI assembly remains unaffected in cells with the m.14430G variant, control cells had significantly higher endogenous and CI-dependent mitochondrial respiration than mutant cells. Accordingly, mutant cells had a lower ATP, MMP and higher extracellular lactate production than control cells. Notably, mutant cells had impaired growth in a galactose-containing medium when compared to wild-type cells. Conclusions: A novel m.14430A > G (p.W82R) variant in the ND6 gene was identified from a patient suspected to have LS, and this variant impaired mitochondrial respiration by decreasing the activity of mitochondrial CI. [ABSTRACT FROM AUTHOR]