Mucins (/ˈmjuːsɪn/) are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals.

[10] Mature mammalian mucins are composed of two distinct regions:[7] The functional classification does not correspond to an exact evolutionary relationship, which is still incomplete and ongoing.

MUC5B, the predominant mucin in the mouth and female genital tract, has been shown to significantly reduce attachment and biofilm formation of Streptococcus mutans, a bacterium with the potential to form cavities.

The dense "sugar coating" of mucins gives them considerable water-holding capacity and also makes them resistant to proteolysis, which may be important in maintaining mucosal barriers.

Upon stimulation, MARCKS (myristylated alanine-rich C kinase substrate) protein coordinates the secretion of mucin from mucin-filled vesicles within the specialized epithelial cells.

The result is a viscoelastic product of interwoven molecules which, combined with other secretions (e.g., from the airway epithelium and the submucosal glands in the respiratory system), is called mucus.

[22][23] Increased mucin production occurs in many adenocarcinomas, including cancers of the pancreas, lung, breast, ovary, colon and other tissues.