We validated Western blot mucin quantification by using tandem mass spectrometry to assess for mucin proteolysis, which could lead to underestimation of the reported mucin concentration as reported in cystic fibrosis.31, 32 Tryptic mapping of MUC5B and MUC5AC showed that peptides were distributed throughout the mucin polypeptides in sputum from all groups, suggesting proteolysis was unlikely (e-Figs 1, 2). Here, MUC5B is linked to cystic fibrosis.