Mucus hypersecretion, a common (chronic bronchitis) phenotype in smokers and individuals with COPD, is associated with increased sputum MUC5AC in smokers and individuals with chronic bronchitis, while emphysema is characterized by high plasma protein levels and components of neutrophil extracellular traps (NETs) in sputum. Here, MUC5AC is linked to chronic obstructive pulmonary disease.