The invention provides cDNA molecules comprising a part of the cDNA sequence of GAD.sub.65 which encode at least one epitope for autoantibodies to GAD.sub.65. The invention also provides cloning vehicles capable of replication and expression comprising cDNA molecules coding for GAD.sub.65. The invention further provides for hosts transformed with a vehicle having a cDNA molecule coding for GAD.sub.65. In another embodiment, the invention provides for the detection of autoantibodies to GAD.sub.65 using the GAD.sub.65 polypeptides coded for by the cDNA molecules of the invention.