However, single fiber analyses of COX−/SDH+ and normal fibers exposed an impairment in mitochondrial biogenesis in COPD; in healthy controls, we detected a marked upregulation of mtDNA copy number and TFAM protein in COX−/SDH+ compared to normal fibers, reflecting the expected compensatory attempt by the oxidative-deficient cells to increase energy levels; in contrast, they were similar between COX−/SDH+ and normal fibers in COPD patients. Here, TFAM is linked to chronic obstructive pulmonary disease.