Of numerous synaptic components that have been identified, perhaps the best-studied are the nicotinic acetylcholine receptors (AChRs) of the vertebrate neuromuscular junction. AChRs are diffusely distributed on embryonic myotubes, but become highly concentrated (approximately 10,000 microns-2) in the postsynaptic membrane as development proceeds. At least two distinct processes contribute to this accumulation. One is local synthesis: subsynaptic muscle nuclei transcribe AChR subunit genes at higher rates than extra-synaptic nuclei, so AChR messenger RNA is concentrated near synaptic sites. Second, once AChRs have been inserted in the membrane, they form high-density clusters by tethering to a subsynaptic cytoskeletal complex. A key component of this complex is rapsyn, a peripheral membrane protein of relative molecular mass 43K (refs 4, 5), which is precisely colocalized with AChRs at synaptic sites from the earliest stages of neuromuscular synaptogenesis. In heterologous systems, expression of recombinant rapsyn leads to clustering of diffusely distributed AChRs, suggesting that rapsyn may control formation of clusters. To assess the role of rapsyn in vivo, we generated and characterized mutant mice with a targeted disruption of the Rapsyn gene. We report that rapsyn is essential for the formation of AChR clusters, but that synapse-specific transcription of AChR subunit genes can proceed in its absence.