A new synapsin protein, designated synapsin III, its amino acid sequence, and its human gene have been isolated and characterized. Furthermore, isoforms of synapsin III, e.g., synapsin IIIa, IIIb and IIIc, and have been isolated and characterized, and cDNA encoding these isoforms has also been isolated and characterized. T he synapin III gene is located on human chromosome 22, in the vicinity of a region previously identified as a susceptibility locus for schizophrenia. The information and experimental tools provided by this discovery can be used to generate new therapeutic agents or diagnostic assays for this new protein, its associated mRNA or its associated genomic DNA. Due to its role in neurotransmission and synaptogenesis, isoforms of synapsin III are associated with the symptoms of psychiatric diseases, especially schizophrenia.