Combined oxidative phosphorylation deficiency 23 (COXPD23, MIM# 616198) is a rare autosomal-recessive mitochondrial disorder with variable disease severity ranging from death in early infancy to survival into the second decade of life,1 with the clinical symptoms of hypertrophic cardiomyopathy (HCM) and/or neurological symptoms with onset in early childhood, hypotonia, delayed psychomotor development, lactic acidosis,1 abnormal lesions in the basal ganglia, thalamus, and brainstem. COXPD23 is caused by homozygous or compound heterozygous mutations in the GTP-binding protein 3 (GTPBP3, OMIM∗ 608536) gene. Except for COXPD23, mutations in GTPBP3 are also associated with other diseases2, 3, 4 (Table S1).