There is provided a polypeptide that binds to syntaxin-1a and comprises the amino acid sequence shown in SEQ ID NO: 1 in the Sequence Listing, or a polypeptide having a homology of at least approximately 90% to the amino acid sequence that binds to syntaxin-1a, a partial peptide thereof, and a polynucleotide encoding any of these polypeptides or the complementary strand thereof, as well as an inhibitor, an antagonist, and an activator of physiological activity of any of the polypeptides. Further, a pharmaceutical composition that utilizes these is provided. There is also provided a method for producing the polypeptides or the peptide by genetic engineering techniques, a method for identifying the inhibitor, antagonist, or activator, and a method and a kit for diagnosing a disease with which the polypeptides are associated.