Fatal infantile cytochrome C oxidase deficiency is a very rare mitochondrial disease characterized clinically by cardioencephalomyopathy resulting in death in infancy. The disease is Monarch Disease Ontology entry MONDO_0015487 (fatal infantile encephalocardiomyopathy). Also known as: cardioencephalomyopathy, fatal infantile, due to cytochrome c oxidase deficiency, fatal infantile COX deficiency, fatal infantile cardioencephalomyopathy due to cytochrome C oxidase deficiency, fatal infantile cardioencephalomyopathy due to cytochrome c oxidase deficiency.