RYR1 and mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria: In this capacity, CcO is an essential enzyme, as evidenced by the plethora of pathologic conditions associated with CcO deficiency, including hearing and visual impairments, exercise intolerance, encephalomyopathy, neuropathy, and cardiomyopathy, among others [3,4,5].