Citrate synthase (CS) activity, a marker of mitochondrial content, was lower in COPD patients (4.6 ± 0.33 μmol per min per g muscle) than in controls (13.4 ± 1.1 μmol per min per g muscle, P < 0.001, Fig. 5e). This evidence concerns the gene CS and chronic obstructive pulmonary disease.