MUC5AC and chronic obstructive pulmonary disease: The two classical clinical phenotypes of COPD are bronchiolitis and emphysema.4 Patients with bronchiolitis display persistent inflammation, goblet cell hyperplasia and mucin hyperexpression in the airway,5 increasing intraluminal mucus, wall muscle fibrosis and airway stenosis.6 Emphysema involves elastolytic destruction of the alveolar wall without obvious fibrosis and loss of normal lung tissue.7 COPD patients with either the bronchiolitis or the emphysema phenotype differ in their clinical characteristics and treatment response.