Cell surface mucins and mucin-like domains comprise a diverse and heterogeneous group of cell surface glycoproteins. The heterogeneity results from both genetic variations in the polypeptides and carbohydrate differences. Mucins form extended rods from the cell surface. The mucin domains apparently serve a protective function, protecting the glycoproteins from cell surface proteolysis and protecting the cells from attack by other cells. Biosynthesis of mucin oligosaccharides is initiated near the transit of the proteins from the endoplasmic reticulum to the Golgi and proceeds rapidly during passage to the cell surface. In some carcinomas a second O-glycosylation pathway adds new oligosaccharides to the glycoproteins after they have reached the cell surface, presumably during recycling.