When present, acetylcholine receptor antibody (AChR) blocks acetylcholine from binding to receptor sites on the muscle membrane. AChR destroys acetylcholine receptor sites, interfering with neuromuscular transmission and causing muscle weakness. Antibodies to acetylcholine receptor sites are present in 90 percent of patients with myasthenia gravis (MG) and in 75 to 80 percent of patients who either have ocular forms of MG or are in remission.