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The 594 amino acid propeptide of glutamate decarboxylase 1 is coded by 16 exons of the GAD1 gene at 2q31.1. It's predominant isoform of four formed by alternative spicing is GAD-67 and is critical to the metabolism of GABA, the major inhibitory neurotransmitter in the vertebral central nervous system. Antibodies to it, are found in a number of autoimmune neurological conditions such as stiff person syndrome and autoimmune epilepsy and encephalitis, but have no specific syndrome associations unlike GAD2[1]. Expression of GAD1 has been implicated in the pathophysiology of schizophrenia[2], in working memory impairment[3] and dyskinesia[4]. The GAD25 isoform is expressed outside the nervous system in pancreatic islets, testis and adrenal cortex.

GAD1 mutation can cause spastic quadriplegic cerebral palsy 1 (CPSQ1, OMIM:603513) with symmetrical, non-progressive spasticity, developmental delay and sometimes epilepsy. Other mutations have been weakly associated with cleft palate.

Glutamate decarboxylases catalyse the breakdown of GABA with pyridoxal 5'-phosphate as a cofactor:

L-glutamate -> 4-aminobutanoate + CO2.