MUSK and myasthenia gravis: Abs targeting AChR, MuSK, or other functionally related molecules (such as the lipoprotein receptor–related protein 4 [LRP4]) induce myasthenia gravis, i.e. a chronic autoimmune disease characterized by weakness of skeletal muscles, which can be generalized or involve only few muscular units, often including the extraocular muscles, with diplopia and ptosis.