Biglycan is a class I small leucine-rich repeat proteoglycan (SLRP) expressed ubiquitously in various extracellular matrix (ECM) tissues, such as bone, cartilage, and tendon. It interacts with and regulates the function of several ECM components, including types I, II, III, and IV collagen, as well as elastin. It is belived to be involved in the mineralization of bone, and, in macrophages, serves as a signal molecule to mediate innate immunity. Compositionally, biglycan is made up of a protein core characterized by leucine-rich repeats, a conserved disulfide loop on the C-terminus, and a cysteine-rich region on the N-terminus. Covalently attached to the amino acid residues at the N-terminus are two highly polar glycosaminoglycan (GAG) chains consisting of either chondroitin sulfate (CS) or dermatan sulfate (DS). GAG chains fill the gaps between collagen fibrils within the ECM by attracting and binding to water molecules providing both lubrication and resistance to comprressive forces. Biglycan is involved in several experimental and human diseases, including Alzheimer's disease, duchene muscular dystrophy, cancer, diabetes, multiple sclerosis, atherosclerosis, asthma, fibrotic liver/kidney disease, intervertebral disc disease, and lupus.