Mucins () are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell signalling to forming chemical barriers. They often take an inhibitory role. Some mucins are associated with controlling mineralization, including nacre formation in mollusks, calcification in echinoderms and bone formation in vertebrates. They bind to pathogens as part of the immune system. Overexpression of the mucin proteins, especially MUC1, is associated with many types of cancer. Although some mucins are membrane-bound due to the presence of a hydrophobic membrane-spanning domain that favors retention in the plasma membrane, most mucins are secreted as principal components of mucus by mucous membranes or are secreted to become a component of saliva. Genes Human mucins include genes with the HUGO symbol MUC 1 through 22. Of these mucins, the following classes have been defined by localization: Secreted mucins: Gel-forming mucins: MUC2, MUC5AC, MUC5B, MUC6, MUC19 Non-gel-forming (soluble) mucins: MUC7 (non-oligomerizing), MUC8, MUC9, MUC20 Membrane-bound (transmembrane) mucins: MUC1, MUC3A, MUC3B, MUC4, MUC12, MUC13, MUC15, MUC16, MUC17, MUC21 (formerly C6orf205), MUC22 (highly polymorphic) The major secreted airway mucins are MUC5AC and MUC5B, while MUC2 is secreted mostly in the intestine but also in the airway. MUC7 is the major salivary protein. Protein structure Mature mammalian mucins are composed of two distinct regions: The amino- and carboxy-terminal regions are very lightly glycosylated, but rich in cysteines. The cysteine residues participate in establishing disulfide linkages within and among mucin monomers. A large central region ("PTS domain") formed of multiple tandem repeats of 10 to 80 residue sequences in which up to half of the amino acids are serine or threonine. This area becomes saturated with hundreds of O-linked oligosaccharides. N-linked oligosaccharides are also found on mucins, but in less abundance than O-linked sugars. Evolutionary classification The functional classification does not correspond to an exact evolutionary relationship, which is still incomplete and ongoing. Known-related groups include: The gel-forming mucins (2, 5AC, 5B, 6, 19) are related both to each other and to otogelin and von Willebrand Factor (PTHR11339). Four of these occur in a well-conserved gene cluster (at 11p.15.5 in humans). The EGF-like domain containing mucins. These include MUC3(A,B), MUC4, MUC12, MUC13, and MUC17. Some EGF-like mucins, plus MUC1 and MUC16, carry SEA domains, a