COPD lower-limb muscles exhibit reduced mitochondrial content and electron-transport/respiratory-chain activity, with decrements in oxidative enzymes such as citrate synthase (CS), succinate dehydrogenase (SDH), and cytochrome c oxidase (COX), mitochondrial DNA (mtDNA) damage, and impaired organelle quality control [15,37,38,39,40,41,42]. The gene discussed is CS; the disease is chronic obstructive pulmonary disease.