Myasthenia gravis (MG) is caused by the failure of neuromuscular transmission mediated by pathogenic autoantibodies (Abs). Generally, patients with MG are divided into 3 groups: (1) nicotinic acetylcholine receptor antibody-positive MG (AChR-MG: 80%), (2) muscle-specific receptor tyrosine kinase antibody-positive MG (MuSK-MG: 5-10%), which are AChR-associated transmembrane post-synaptic proteins involved in AChR aggregation, and (3) double-seronegative MG. In 2011, autoantibodies against low-density lipoprotein receptor-related protein 4 (Lrp4) were identified in Japanese MG patients, and thereafter, have been reported in Germany and USA. We developed a simple technique termed Gaussia luciferase immunoprecipitation (GLIP) for detecting the antibodies to Lrp4. Our results showed that 9 generalized MG patients out of 300 without AChR Ab were positive for Lrp4 antibodies. These antibodies inhibit the binding of Lrp4 to its ligand and predominantly belong to the IgG1 subclass. In other studies, Lrp4 Ab-positive sera inhibited agrin-induced aggregation of AChRs in cultured myotubes, suggesting a pathogenic role in the dysfunction of the neuromuscular endplate. Further understanding of the structure and function of neuromuscular junction (NMJ) through newly discovered autoantibodies may provide specific clinical information and treatment for MG.