Mucins are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals. Mucins are distinguished between membrane-bound mucins, forming the glycocalyx, or secreted mucins, forming mucus-gels. These mucus gels cover the wet epithelium, serving functions from lubrication, to physico-chemical barrier formation, protecting against foreign particles and pathogens as seen in e.g. the lungs or intestine. Next to their physical properties, gel-forming and transmembrane mucins alike are also involved in cell signalling and immune responses. 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.