As the key component of the cartilage extracellular matrix, aggrecan hydrates the collagen network and provides cartilage with its properties of compressibility and elasticity. Maintenance of aggrecan content is therefore critical to the function of the tissue and aggrecan degradation is an important factor in the erosion of articular cartilage in arthritic diseases. The deduced amino acid sequence of human aggrecan core protein consists of 2415 residues and predicts a signal peptide and domains of G1, IGD, G2, KS, CS­1, CS­2 and G3 (3). Two globular domains, G1 and G2, comprise the N­terminus of the proteoglycan and also contain link domains. The third globular domain, G3, corresponds to the C­terminus. The keratan sulfate (KS) and the chondroitin sulfate (CS) attachment domains are between G2 and G3. With KS and CS attached to the 250kD core protein, aggrecan monomers exist as a 1000-2000kD molecule.