CoQ (ubiquinone) serves as a redox carrier in the mitochondrial respiratory chain and is a lipid-soluble antioxidant. CoQ10 deficiency has been associated with an enlarging spectrum of phenotypes (Table 56.1). Many patients had cerebellar ataxia, some had recurrent myoglobinuria with ragged-red fibers and lipid storage in muscle, and others had infantile encephalomyopathy, lactic acidosis, hypertrophic cardiomyopathy, optic atrophy, cataracts, sensorineural hearing loss, and nephropathy with focal and segmental glomerulosclerosis. In a boy with infantile encephalomyopathy and nephropathy, and deficiency of CoQ10 in muscle and fibroblasts, a homozygous p.Y297C mutation in the COQ2 gene was found. In fibroblasts carrying the same mutation, there was decreased ATP synthesis and increased oxidative stress-induced death, suggesting that partial CoQ10 deficiency can cause oxidative stress that is toxic to the cells.