A 59-year-old male with diabetes mellitus, diabetic polyneuropathy, RNS-confirmed AChR antibody negativity, and late-onset generalized myasthenia gravis (LOMG) was also diagnosed with anti-GAD65 antibody-positive stiff-person syndrome, affecting the distal upper extremities and anterior tibialis muscles. His CK levels were elevated (393–544 U/L). A chest CT scan revealed thymic hyperplasia. Serological testing for anti-muscle antibodies was negative. The gene discussed is GAD2; the disease is diabetic polyneuropathy.