Myasthenia gravis (literally "serious muscle-weakness"; from Greek μύς "muscle", ἀσθένεια "weakness", an Latin gravis "serious"; abbreviated MG) is a neuromuscular disease leading to fluctuating muscle weakness and fatiguability. It is an autoimmune disorder, in which weakness is 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 medically with cholinesterase inhibitors or immunosuppressants, and, in selected cases, thymectomy. At 200-400 cases per million it is one of the less common autoimmune disorders.