GAD1 and type 1 diabetes mellitus: The closely related 67 kDa isoform, GAD67, is 71% identical in its amino acid sequence but is rarely an autoantigen in T1D [1], [10], [11], interacts differently with the pyridoxal-5′-phosphate (PLP) co-factor, and has different kinetics for GABA synthesis in enzyme activity assays [12].