The term “mucin” has changed its meaning over the last decades prompted by the impressive progress made in the field of glycobiology. Up to the 1970s researchers exclusively used this term to refer to the major glycoprotein components in secreted mucus lining the surfaces of glandular epithelia. The best characterized species during that time, the mucins from ovine and bovine submaxillary glands, served as structural models for this subclass of glycoproteins, since they exhibited the features regarded as typical for mucins: a high carbohydrate content (exceeding 50% by weight) with a concomitant high buoyant density and a threonine/serine rich peptide core serving as a scaffold for the addition of uniform, simple and mainly acidic oligosaccharides. A dense hydrophilic coat of O-linked negatively charged glycans was a simple structural model, but in accord with the proposed function of mucins which was regarded to lie in the formation of a viscoelastic gel serving in physicochemical protection of epithelial surfaces. Two important observations, which were made during the 1980s, have changed this view and conferred the mucins increased attraction as a research topic. Using advanced methodology and sophisticated instrumentation (FABMS, 500 MHz H-NMR) several groups could demonstrate by means of structural chemistry that mucins are much more complex glycosylated than expected (Lamblin et al., 1984; Hanisch et al., 1985, 1986; Hounsell et al., 1985, 1989; Mutsaers et al., 1986). A second major point was the identification of tumor-associated epitopes on mucins as immunotargets on malignant epithelial cells and their secretions (Magnani et al., 1983; Hilkens et al., 1984; Burchell et al., 1987). In particular the latter aspect has driven tremendous efforts to characterize distinct mucin species by recombinant technology. In 1990 four groups were able to sequence the first human mucin gene on the DNA level (Gendler et al., 1990; Lan et al., 1990; Ligtenberg et al., 1990; Wreschner et al., 1990). Designated as

MUC1 in accord with the Human Genome Mapping conventions, this mucin protein is identical to the polymorphic epithelial mucin (PEM), the polymorphic urinary mucin (PUM), Episialin, DF3 antigen, and several other glycoforms of MUC1 isolated from various sources. MUC1 and meanwhile a series of other human mucins (MUC2 to MUC12) have revealed to exhibit large domains of tandemly repeated peptides as a structural characteristic of the “real” mucins (Porchet et al., 1991;