Extracellular MPO activity levels in lung tissues from elastase-exposed Duox1–/– mice were significantly increased compared with corresponding WT mice, whereas intracellular MPO activity levels were unaffected (Figure 6, E and F), suggesting that Duox1 deficiency promotes neutrophil activation and degranulation in this model of elastase-induced emphysema. Here, DUOX1 is linked to pulmonary emphysema.