AGRN and rheumatoid arthritis: Of the seven identified proteins are, Agrin (AGRN), an extracellular matrix heparan sulfate proteoglycan (HSPG) with many distinct structural domains70, is detected in the cartilage of healthy patients, but its expression progressively decreases in chondrocytes of OA patients71, though it is upregulated in the synovial fluid of both OA and RA patients.