COMP/Thrombospondin-5 is a multidomain calcium binding matricellular protein. Thrombospondin-1 and -2 form homotrimers, whereas Thrombospondin-3, -4, and COMP form homopentamers. COMP mediates chondrocyte-matrix adhesion via Integrin binding. COMP is upregulated in rheumatoid arthritis and osteoarthritis, hepatocellular carcinomas, chronic pancreatitis, and pancreatic carcinomas. Mutations in COMP are associated with skeletal dysplasias and disorders of calcium metabolism.