Gel-forming mucin that is thought to contribute to the lubricating and viscoelastic properties of whole saliva and cervical mucus. Expressed on surface airway epithelia. Expressed mainly in mucous cells of submucosal glands of airway tissues. Highly expressed in the sublingual gland. Also found in submaxillary glands, endocervix, gall bladder, and pancreas. Regulated by all-trans-retinoic acid (ATRA) in a cell-type specific manner. The cysteine residues in the Cys-rich subdomain repeats are not involved in disulfide bonding. Highly glycosylated. C-, N- and O-glycosylated. C-mannosylated in the Cys-rich subdomains probably on the first Trp residue of the WXXW motif. Highly O-glycosylated in the Ser/Thr-rich tandem repeat (TR) region. The repeat region is about 59% O-glycosylated with a high abundance of NeuAc(2)Hex(1)HexNac1-ol. Disease susceptibility is associated with variants affecting the gene represented in this entry. A common polymorphism in the promoter of MUC5B is associated with familial interstitial pneumonia and idiopathic pulmonary fibrosis, suggesting that dysregulated MUC5B expression in the lung may be involved in the pathogenesis of pulmonary fibrosis (PubMed:21506741). Truncated N-terminus.