Myasthenia Gravis (MG) is caused by auto-antibodies to the nicotinic acetylcholine receptor (AChR). Rapid progress has been made recently in the study of both the disease and the AChR. A significant part of this progress can be attributed to monoclonal antibodies (mAbs) raised against the AChR. These antibodies have been used to identify, purify and study the structure, function and biosynthesis of AChR from electric organs, muscles and neurons. In MG, these mAbs have been used as model auto-antibodies in in vivo and in vitro studies. They have also been used as tools for the structural and functional analysis of the patients' antibodies. A region on a small segment of the AChR alpha-subunit (main immunogenic region) dominates the immunogenicity and probably the pathogenicity of the molecule.