A heparan sulphate proteoglycan released by motor neurons and muscle cells that is crucial for the normal development or regeneration of neuromuscular junctions. As a growing axon makes contact with a muscle cell, the agrin acts as a signal to recruit essential components and start the formation of a synapse between the axon and muscle. Agrin binds to its receptor, called muscle-specific receptor tyrosine kinase (MuSK), in the muscle cell membrane and triggers a cascade of events that results in the accumulation of acetylcholine receptors beneath the muscle cell membrane. Binding of agrin to other receptors, such as laminin β‎2, is also required to stabilize the newly forming synaptic structure. Agrin is equally important in the natural regeneration of damaged neuromuscular junctions and also seems to have a role in the differentiation of presynaptic nerve terminals.