Keratan sulfate is any of several sulfated glycosaminoglycans that have been found especially in the cornea, cartilage, and bone.  Keratan sulfates are large, highly hydrated molecules which in joints can act as a cushion to absorb mechanical shock. The designations KSI and KSII were originally assigned on the basis of the tissue type from which the keratan sulfate was isolated. KSI was isolated from corneal tissue and KSII from skeletal tissue. Minor monosaccharide compositional differences exist between KS extracted from both sources and even KS extracted from the same source. KSI is N-linked to specific asparagine amino acids via N-acetylglucosamine and KSII is O-linked to specific A third type of KS (KSIII) has also been isolated from brain tissue that is O-linked to specific serine or threonine amino acids via mannose.