GAD1 (Glutamate decarboxylase 1, GAD67, brain, 67kDa) is a glutamic acid decarboxylase that functions to catalyze gamma-aminobutyric acid (GABA) synthesis from L-glutamic acid. It is an autoantigen in insulin-dependent diabetes, and mutation or deficiency in GAD1 results in pyridoxine-dependent seizures. GAD1 mutation or dysregulation are also thought to contribute to improper neuronal activity and connection formation during development and may be involved in cognitive dysfunction in schizophrenia. Separately, epigenetic upregulation of GAD1 leads to an increase in aggressive characteristics in metastatic brain cancers. GAD1 has highest positivity in the cerebral cortex, hippocampus, cerebellum and basal ganglia in the brain, with limited expression in the testes, parathyroid and a few other tissues. References: PLoS One. 2017 Jan 25;12(1):e0170805, PMID: 28122016; Sci Rep. 2018 Oct 19;8(1):15470, PMID: 30341396; Cancer Res. 2017 Jun 1;77(11):2844-2856, PMID: 28400476; Neurology. 2000 Jul 25;55(2):309-11, PMID: 10908915