In contrast, MUC5AC appears to be expressed predominantly in the proximal airway and is induced by a variety of triggers such as infection and cigarette smoke, as well as in allergic (type 2) inflammation, for example asthma.18 Cells either exclusively produce MUC5AC or MUC5B but not both, and MUC5B remains the predominant mucin throughout the airway.17 The gene discussed is MUC5B; the disease is asthma.