In five members of a family and another unrelated person affected by a slow-channel congenital myasthenic syndrome (SCCMS), molecular genetic analysis of acetylcholine receptor (AChR) subunit genes revealed a heterozygous G to A mutation at nucleotide 457 of the a subunit, converting codon 153 from glycine to serine (aG153S). Electrophysiologic analysis of SCCMS end plates revealed prolonged decay of miniature end plate currents and prolonged activation episodes of single AChR channels. Engineered mutant AChR expressed in HEK fibroblasts exhibited prolonged activation episodes strikingly similar to those observed at the SCCMS end plates. Single-channel kinetic analysis of engineered aG153S AChR revealed a markedly decreased rate of ACh dissociation, which causes the mutant AChR to open repeatedly during ACh occupancy. In addition, ACh binding measurements combined with the kinetic analysis indicated increased desensitization of the mutant AChR. Thus, ACh binding affinity can dictate the time course of the synaptic response, and aG153 contributes to the low binding affinity for ACh needed to speed the decay of the synaptic response.