Anti-GAD antibodies target an enzyme called glutamic acid decarboxylase. This enzyme is responsible for converting glutamic acid to GABA, a chemical found in high concentrations in the cerebellum. It is believed that the lack of GABA results in cerebellar ataxia. Patients with cerebellar ataxia of an unknown cause should have an anti-GAD test.

The anti-GAD antibodies have also been associated with a disease characterized by stiffness of the muscles, called stiff person syndrome. The stiff person syndrome and cerebellar ataxia do not necessarily occur together in patients with anti-GAD antibodies. Anti-GAD antibodies are particularly common in diabetes mellitus, as autoimmune diseases tend to occur together. The treatment for anti-GAD antibodies is corticosteroids or prednisone to reduce the abnormal immune response.