Another type of cellular glycan is the glycosaminoglycans (GAGs). These comprise 2-aminosugars linked in an alternating fashion with uronic acids, and include polymers such as heparin, heparan sulfate, chondroitin, keratan and dermatan. Some glycosaminoglycans, such as heparan sulfate,  are found attached to the cell surface, where they are linked through a tetrasacharide linker via a xylosyl residue to a protein (forming a glycoprotein or proteoglycan).