GAD1 and type 1 diabetes mellitus: T1D prediction and diagnosis have been established for decades and are based on the detection of autoantibodies (aAbs) directed against β-cell proteins such as insulin (so-called insulin aAbs, IAA), glutamic acid decarboxylase (GAD), insulinoma-associated protein 2 (IA-2), and/or zinc transporter 8 (ZnT8) [7], which precede clinical symptoms by years.