In recent years considerable advances have been made in our knowledge of the peptide moiety of human mucins through cDNA cloning. In many diseases disorders in mucin biosynthesis are observed, which result either from changes in the synthesis of the carbohydrate side chains or from differences in the relative expression of the different apomucins, each of which may affect physical properties of the viscous gel. We describe in situ hybridization studies on healthy human mucosae with five different oligonucleotide probes corresponding to each of the human genes known to date that encode secreted mucins, i.e., MUC 2, 3, 4 (HGM nomenclature) and 5B, 5C (proposed name). These genes present a nucleic tandem repeat organization. The choice of oligonucleotide probes was made to amplify the signal by hybridization of many small probes on the same mRNA molecules. A characteristic pattern of mucin gene expression was observed for each mucosa.

The cells of living organisms in contact with the external environment are constantly attacked by different kinds of substances such as micro-organisms, toxins, and pollutants. With evolution, defense mechanisms, such as the secretion of mucus has been developed. Mucins are the main components of mucus. They are synthesized and secreted by specialized cells of the epithelium and in some case, by non mucin-secreting cells. Little was known about the structure of mucins until a decade ago. This is principally due to heavy glycosylation of mucins, which complicated their analysis. With the application of molecular biological methods, structures of the mucin core peptides (apomucins) are beginning to be elucidated. A total of eleven human mucin (MUC) genes have been identified and numbered in chronological order of their description: MUC1-4, MUC5AC, MUC5B, MUC6-8, and MUC11-12. Of these, the complete cDNA sequence are published only for six mucins MUC1, MUC2, MUC4, MUC5B, MUC5AC, and MUC7. Human mucin genes, in general, show three common features: I) a nucleotide tandem repeat domain; II) a predicted peptide domain containing a high percentage of serines and threonines; III) complex RNA expression. The tandem repeats in mucins make up the majority of the backbone. Related to their structure, mucins can be classified in three distinct sub-families: gel-forming, soluble, and membrane-bound. Each member from one family possesses common characteristics and probably specific functions. For a long time, they were thought to have the unique function of protecting and lubricating the epithelial surfaces. The study of the mucins structure as well as the relationship between structure and function show that mucins also possess other important functions, such as growth, direct implication in the fetal development, the epithelial renewal and differentiation, the epithelial integrity, carcinogenesis, and metastasis. This review presents the actual knowledge on the mucins structure and the best-characterized function related to their structure.

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