The ETC, mitochondrial membrane permeability, and apoptotic signalling all seem to be affected in COPD: increased cytochrome oxidase activity and citrate synthase in the vastus lateralis muscle and upregulation of activities of other membrane-bound respiratory chain enzymes and abnormal mitochondrion permeability transition pore kinetics and cytochrome c release in skeletal and respiratory muscles in patients with COPD were reported [187–190]. Here, CYCS is linked to chronic obstructive pulmonary disease.