ETC complex protein I (NADH dehydrogenase) protein expression was reduced in mitochondria isolated from COPD patient biopsies compared to healthy (− 85.1%, p < 0.01) and ex-smoker (− 42.2%, p < 0.01) groups (Fig. 2a). This evidence concerns the gene ENSG00000251605 and chronic obstructive pulmonary disease.