Another protein that is produced by islet β-cells as well as many CNS neurons, glutamic acid decarboxylase (GAD), is an important autoantigen in both IDDM and stiff-man syndrome, an uncommon CNS disorder caused by impaired GABAergic neurotransmission that often coexist with IDDM [29,30]. Here, GAD1 is linked to stiff-person syndrome.