Agrin is a large proteoglycan whose best characterised role is in the development of the neuromuscular junction during embryogenesis. Agrin is named based on its involvement in the aggregation of acetylcholine receptors during synaptogenesis. This protein has nine domains homologous to protease inhibitors. It may also have functions in other tissues and during other stages of development. It is major proteoglycan component in glomerular basement membrane and may play a role in the renal filtration and cell-matrix interactions. This protein binds to several receptors on the surface of skeletal muscle. The receptor which seems to be required for formation of the neuromuscular junction (NMJ) is called the MuSK receptor (Muscle specific kinase).