Cytochrome c oxidase assembly protein COX15 homolog (COX15), also known as heme A synthase, is a protein that in humans is encoded by the COX15 gene.[5][6] This protein localizes to the inner mitochondrial membrane and involved in heme A biosynthesis.[7] COX15 is also part of a three-component mono-oxygenase (ferredoxin, ferredoxin reductase, and COX15) that catalyses the hydroxylation of the methyl group at position eight of the protoheme molecule.[7] Mutations in this gene has been reported in patients with hypertrophic cardiomyopathy as well as Leigh syndrome, and characterized by delayed onset of symptoms, hypotonia, feeding difficulties, failure to thrive, motor regression, and brain stem signs