MUC5AC and chronic obstructive pulmonary disease: Defective mucus hydration, as a result of a primary defect in epithelial ion transport or due to hypersecretion of “dry” mucin macromolecules, has been established as a key pathomechanism in a spectrum of muco-obstructive lung diseases, such as CF, chronic obstructive pulmonary disease (COPD), and asthma (Button et al., 2016).