MUSK and myasthenia gravis: Myasthenia gravis (MG) is the main representative of this group and is a chronic autoimmune disease that involves the dysfunction of neuromuscular junctions mediated by antibodies against proteins of the postsynaptic membrane, including the acetylcholine receptor (ACh-R) [3], muscle-specific kinase (MuSK) [4] and low-density lipoprotein receptor-related protein 4 (LRP4) [5].