In order to understand the reasons for increased mucin stability at the start of an exacerbation, we quantified NE and A1-PI in sputum samples using an immunoblot and found that the NE concentration in the sputum of the COPD patients at the onset was 3.5 times lower than at 5–6 weeks after the onset of the exacerbation (Fig 2a). Here, MUC5AC is linked to chronic obstructive pulmonary disease.