Proteolytic cleavage of MUC5B has been reported in CF (6, 9); to assess whether this occurs during synthesis, we employed the tryptic peptide mapping approach outlined to analyze glycosylated MUC5B isolated by CsCl density gradient centrifugation (by the 2-step approach shown in Fig. 5) of cell lysates or apical secretions from CF airway cells cultured at ALI (data not shown). Here, MUC5B is linked to cystic fibrosis.