Notably, bi‐allelic COX18 pathogenic variants have been molecularly linked to severe mitochondrial disorders characterised by progressive encephalomyopathy and neuropathy, as demonstrated in recent clinical case reports with biochemical validation of respiratory chain deficiencies.33, 34, 35. The gene discussed is COX18; the disease is mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria.