MuSK (muscle-specific kinase) is a 100kD type I transmembrane (TM) protein belonging to the receptor tyrosine kinase family. It is found in the postsynaptic membrane of skeletal muscle motor endplates. MuSK does not bind agrin directly, but is part of an agrin receptor complex that promotes the clustering of acetylcholine receptor at the neuromuscular junction (NMJ). Over residues 22-503, human MuSK shares 92% amino acid sequence identity with mouse MuSK. Human MuSK apparently has multiple isoforms. One contains deletions at residues 307-394 and 454-461, while a second is a short soluble form that contains residues 120-209 plus a unique 24 aa C-terminal tail.