Two main subfamilies of the mucin glycoproteins have been identified: secreted and membrane-bound. The secreted mucins can be further divided into insoluble gel-forming mucins, including MUC2, MUC5AC, MUC5B, MUC6 and MUC19, and soluble mucins, including MUC7, MUC8 and MUC9. Evolutionary studies showed that the gel-forming mucins are more ancient than the membrane-bound mucins. The evolutionary separation of these two subfamilies is partially reflected in the chromosomal localization of the genes encoding each of mucins. The differences between secreted and membrane-bound mucins are also reflected in the composition of their structural domains, in biosynthesis of their precursors and in posttranslational modifications. Despite some differences, the common features of mucin glycoproteins, such as the structure of the mucin specific domain with its tandem repeats and associated functions, relate them to the same protein family.