COQ9 and coenzyme Q10 deficiency: To date, pathogenic mutations in 10 genes (PDSS1, PDSS2, COQ2, COQ4, COQ6, COQ7, COQ8A/ADCK3, COQ8B/ADCK4, and COQ9) involved in the biosynthesis of CoQ10 have been reported to cause primary CoQ10 deficiency, which has variable clinical symptoms ranging from fatal neonatal multisystem disorder to adult-onset encephalopathy or nephropathy (Desbats et al., 2015; Acosta et al., 2016).