A cDNA encoding for rat synapsin I, a neuron-specific protein localized on the cytoplasmic surface of synaptic vesicles and probably involved in the regulation of neurotransmitter release from nerve terminals, has been used to map the human gene to the short arm of the X chromosome. We have screened, using this cDNA, the DNAs of six unrelated girls with the Rett Syndrome (RS) to test the hypothesis that mutations of the human synapsin I gene might cause RS. We found no alterations at the synapsin I (Syn I) locus in the vicinity of the probe sequence.