Neurotransmission takes place at synapses, highly specialized sites of contact between neurons. Transmission across synapses occurs through a variety of mediators including amino acid neurotransmitters, peptides, electrical current, and even gaseous messengers. Processing of these signals is carried out by receptors, ion channels, and signaling molecules that are clustered at synapses. However, the molecular mechanisms that assemble these components at synapses remain largely unknown. Some insight into these mechanisms has been revealed from studies of the neuromuscular junction (NMJ), a prototypical chemical synapse where release of acetylcholine from motor neurons signals muscle contraction. During muscle development nicotinic acetylcholine receptors (nAChRs) become clustered at the muscle endplate in response to signals from the presynaptic motor neuron. A 43 kDa protein of the muscle cytoskeleton, rapsyn, has been shown by gene disruption to be required for the clustering of nAChRs and may anchor the receptors to a subsynaptic cytoskeletal complex (