Biallelic loss‐of‐function variants in COQ9 cause Primary Coenzyme Q10 deficiency 5 (COQ10D5; OMIM: 614654), a multiorgan condition with features including intrauterine growth retardation (IUGR), microcephaly, neurological and cardiovascular disease, and metabolic lactic acidosis. The gene discussed is COQ9; the disease is Encephalopathy - hypertrophic cardiomyopathy - renal tubular disease.