Several bacterial MT-CO2 have a C-terminal extension that contains a covalently bound haem c.[10][11] The MT-CO2 gene encodes for the second subunit of cytochrome c oxidase (complex IV), a component of the mitochondrial respiratory chain that catalyzes the reduction of oxygen to water.

Other Clinical Manifestations include hypertrophic cardiomyopathy, hepatomegaly and liver dysfunction, hypotonia, muscle weakness, exercise intolerance, developmental disability, delayed motor development and mental retardation.

A deletion mutation of a single nucleotide (7630delT) in the gene has been found to cause symptoms of reversible aphasia, right hemiparesis, hemianopsia, exercise intolerance, progressive mental impairment, and short stature.

[16] Furthermore, a patient with a nonsense mutation (7896G>A) of the gene resulted in phenotypes such as short stature, low weight, microcephaly, skin abnormalities, severe hypotonia, and normal reflexes.

[17] A novel heteroplasmic mutation (7587T>C) which altered the initiation codon of the MT-CO2 gene in patients have shown clinical manifestations such as progressive gait ataxia, cognitive impairment, bilateral optic atrophy, pigmentary retinopathy, a decrease in color vision, and mild distal-muscle wasting.