MuSK (for Muscle Specific Kinase) is a receptor tyrosine kinase required for the formation of the neuromuscular junction (NMJ). It induces cellular signaling by causing the addition of phosphate molecules to particular tyrosines on itself, and on proteins which bind the cytoplasmic domain of the receptor. It is activated by a nerve-derived proteoglycan called agrin. During development, the growing end of motor neuron axons secrete a protein called agrin.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), which comprises the nerve-muscle synapse is called MuSK. MUSK mutations lead to decreased agrin-dependent AchR aggregation, a critical step in the formation of the neuromuscular junction.