Coenzyme Q₁₀ is an essential cofactor for mitochondrial function and other biochemical pathways. Mutations in genes involved in CoQ₁₀ biosynthesis cause primary CoQ₁₀ deficiency syndromes that can be treated with oral ubiquinone. We used exome sequencing to evaluate 6 patients with clinical findings suggestive of a mitochondrial disorder. All patients were female and presented on the first day of life and died in the neonatal period or early infancy. Other clinical findings included hypotonia, encephalopathy, neonatal seizures, cerebellar atrophy, cardiomyopathy, and lactic acidosis. We identified recessive mutations in COQ4 gene in all patients studied; one was a founder mutation in the Ashkenazi Jewish population (By Marwan Shinawi, M.D., http://jmg.bmj.com/content/early/2015/07/16/jmedgenet-2015-103140 ).