The characteristics of Rett syndrome suggest that it is an X-linked neurodegenerative disorder. Laboratory investigations to date have not revealed any metabolic abnormalities in affected individuals. Synapsin I is a neuron-specific protein thought to play a fundamental role in neuronal function. In this report we summarize the circumstantial evidence suggesting that a defect in synapsin I gene structure or expression might be involved in Rett syndrome. This evidence includes analysis of structural and functional aspects of synapsin I primary structure, characterization of synapsin I messenger RNAs, location of the synapsin I gene on the human X chromosome and preliminary analysis of synapsin I gene structure in Rett individuals.