A class of highly sialylated glycoproteins with very large mucin-like domains that protrude high above the plasma membrane have been shown to strongly reduce cellular adhesion. In normal epithelial cells, where the expression is restricted to the luminal side of the cell, these molecules may prevent inadvertent closing of the lumen as a result of weak, non-specific protein-protein interactions between opposite luminal membranes. In malignant tumors cellular polarization is often lost, which can lead to the entire cell surface being covered by these molecules. The resulting strongly reduced adhesion and immune recognition properties may play an important role during invasion and metastasis.