Agrin is a molecule that resides in the basal lamina of muscle cells and directs key events in post synaptic differentiation. Most notably, Agrin is responsible for the clustering of acetylcholine receptors (AChRs) on the cell surface and their localization to the neuromuscular junction. Several Agrin variants have been identified which arise from alternative mRNA splicings. Agrin splice forms having inserts at two sites in the carboxy terminus designated “y” and “z” display a high affinity for AChRs, while splice forms lacking these inserts associate with AChRs weakly. Muscle alpha-dystroglycan has been postulated to be the receptor for the clustering activity of agrin; however, this is a point of contention. Tyrosine phosphorylation has been implicated as a required early step in AChR aggregation. Interestingly, a unique receptor tyrosine kinase, designated MuSK, has been discovered that interacts with Agrin and is specifically localized to developing muscle .