Abstract: The present invention provides a chimeric polypeptide comprising an epitope of GAD65 protein and a structural region comprising a polypeptide of the GAD family, wherein the chimeric polypeptide is a more specific diagnostic for insulin dependent diabetes mellitus than intact GAD65 and produces fewer false positives than intact GAD65. The invention further provides a method of screening a subject for risk of developing IDDM, comprising contacting the chimeric polypeptide of claim 1 with a biological sample containing antibodies from the subject and detecting binding between an antibody in the biological sample and the chimeric polypeptide, the detection of binding indicating the subject is at risk of developing IDDM.