Rapsyn, a complex postsynaptic protein of the striated muscle, assembles acetylcholine receptors (AChR) at high density at the motor endplate (EP). Neuromuscular junctions of mice lacking rapsyn show no clusters of AChRs or other structural postsynaptic proteins such as beta-dystroglycan and utrophin. Humans with mutations in the rapsyn gene ( RAPSN) are affected with a postsynaptic form of congenital myasthenic syndrome (CMS) characterized by impairment of the morphologic development of the postsynaptic region. We have identified four patients from four different families with RAPSNmutations and CMS, confirmed in two cases by microelectrode and electron microscopy studies. The N88K mutation was present in all patients. One patient who was homozygous for N88K was only mildly affected, while the other three patients who were heterozygous for N88K and a second mutation (either L14P, 46insC, or Y269X) were severely affected. Mutations 46insC and Y269X predicts truncation of the protein. L14P predicts a conformational change at the N-terminus that may disrupt membrane association. N88K occurs within the putative leucine zipper motif potentially important for AChR clustering. These findings may explain the severe clinical involvement of compound heterozygous patients.