MUC5AC and cystic fibrosis: Further investigation showed that human neutrophil elastase, a potent protease found in the inflamed airways of patients with CF [61], VAP [62], COPD, severe asthma, and bronchiectasis [63], could cleave the exposed protein region of MUC5AC and MUC5B, accounting for the lower mucin concentrations in CF sputum when immunological techniques were used [61].