In muco-obstructive airway diseases such as asthma, COPD, cystic fibrosis (CF), and non-CF bronchiectasis, increased MUC5AC production in response to inflammatory stimuli leads to extensive changes of the airway epithelium, termed mucous metaplasia (MM) marked by enlargement of secretory cells with increased intracellular staining of MUC5AC granules. The gene discussed is MUC5AC; the disease is cystic fibrosis.