Myasthenia gravis (sometimes abbreviated MG; from the Greek myastheneia, lit. 'condition of no strength in the muscle', and Latin gravis, 'serious') is a neuromuscular disease leading to fluctuating muscle weakness and fatiguability. At 20 cases per 100,000 (in the U.S.), it is one of the lesser known autoimmune disorders. Weakness is typically caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy.