MUC5B is a gel-forming mucin that contributes to the lubricating and viscoelastic properties of airway secretions 1. In human bronchial airways, MUC5B is exclusively expressed in mucous tubules of submucosal glands 1 and is strongly induced during differentiation of airway epithelia 2. MUC5B and MUC5AC are the predominant polymeric mucins in mammalian airways, variably packaged together or separately in secretory granules 3. MUC5B plays distinct roles in mucociliary transport, mobilizing mucus to enable its active clearance by cilia, whereas MUC5AC ensures spatial alignment of this transport 4. A common promoter polymorphism (rs35705950-T) located 3 kb upstream of the MUC5B transcription start site is strongly associated with idiopathic pulmonary fibrosis (IPF) and familial interstitial pneumonia, with odds ratios of 9.0-20.8 5. This variant associates with increased MUC5B expression in lung tissue 5, and MUC5B protein is detected in IPF lesions 5. The rs35705950-T allele also associates with reduced COVID-19 hospitalizations 6. Dysregulated MUC5B expression contributes to mucus hypersecretion in chr11 obstructive pulmonary disease, suggesting therapeutic potential for agents like N-acetylcysteine that inhibit MUC5B expression 7.