Mice with a truncating R239X mutation in the Coq9 gene [54]: • impaired mitochondrial respiration with loss of ATP and complex I activity, • encephalomyopathy, • neuronal death, demyelination, vacuolization, spongiform degeneration, and astrogliosis, • cardiac fibrosis, • impaired locomotor activity and progressive paralysis, • early mortality. Here, COQ9 is linked to mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria.