Mucins are high molecular mass, highly glycosylated macromolecules that are the major components of mucus secretions. MUC5B is a salivary mucin that is thought to contribute to the lubricating and viscoelastic properties of whole saliva. It is composed of 14.9% protein, 78.1% carbohydrate, and 7% sulfate.Desseyn et al. isolated MUC5B genomic sequence. They found that the central exon encodes a predicted 3,570-amino acid peptide containing 19 subdomains that can be grouped into 4 larger composite units referred to as 'superrepeats'. Since MUC5B and the related proteins MUC2 and MUC5AC are clustered within chromosome 11p15.5, suggesting that they are members of a multigene family.Desseyn et al. reported that the region 3-prime of the large central exon of MUC5B is composed of 18 exons and spans about 10.7 kb.