COMP (Cartilage Oligomeric Matrix Protein), also known as Thrombospondin-5, is a multidomain calcium binding protein that associates with other extracellular matrix molecules. COMP associates into disulfide-linked homopentamers with a central hub and peripheral globular domains connected by flexible strands. An axial pore is formed by the coiled coil assembly and binds vitamin D3 which is involved in bone and cartilage metabolism. An RGD sequence in the third TSP type-3 repeat mediates chondrocyte attachment via Integrins a5b1 or aVb3. COMP is upregulated in rheumatoid arthritis and osteoarthritis, hepatocellular carcinomas, chronic pancreatitis, and pancreatic carcinomas. Several mutations in COMP are associated with skeletal dysplasias.