GAD1 and type 1 diabetes mellitus: GABA and the enzymes essential for its production and breakdown (i.e., glutamic acid decarboxylase [GAD] and GABA transaminase [GABA-T]) are present in the beta cells of pancreatic islets [59,60,61,62], and the presence of plasma anti-GAD autoantibodies is a hallmark of T1D [63,64].