The presence of high-titer antibodies against glutamic acid decarboxylase (GAD) in the patients’ serum, also reflecting intrathecal synthesis, defines the SPS-spectrum disorders (SPS-SD or GAD-SD) that include stiff-person syndrome, cerebellar ataxia, autoimmune epilepsy, and encephalopathy [131–133]. This evidence concerns the gene GAD1 and stiff-person syndrome.