Membrane-associated and secretory mucins are high molecular weight glycoproteins of the glycocalyx (polysaccharide biofilm) that protects mucosal epithelium from particulate matter and microorganisms. The mucin family consists of Mucins 1-4, Mucin 5 (AC and B), Mucins 6-8, Mucins 11-13 and Mucins 15-17. A family of four related Mucin genes (MUC2, MUC5AC, MUC5B and MUC6) encode the major secreted mucins. Mucin 12 contains a predicted transmembrane domain; two extracellular cysteine-rich EGF-like domains; a coiled-coil region; and a domain consisting of serine-, threonine-, and proline-rich degenerate tandem repeats of 28 amino acids, a structural feature typical of mucins. Mucin 12 transcript (>12 kb) is present in colon, pancreas, prostate and uterus. Colorectal tumors can have low Mucin 12 transcript levels in comparison to normal colon tissues.