Aggrecan degradation is an important factor in the erosion of articular cartilage in arthritis. Aggrecanase is a newly cloned enzyme which degrades the core protein of aggrecan between the Glu³⁷³ and Ala³⁷⁴, a site different from the matrix metalloproteinases cleavage site, Asn³⁴¹～Phe³⁴². The discovery of aggrecanase and searching for its inhibitors will accelerate the development of therapeutic agents for arthritis.