MUSK and myasthenia gravis: Most patients with MG have autoantibodies against the acetylcholine receptors (AChRs) [2,3], and a minority are seropositive for antibodies directed to muscle-specific kinase (MuSK) [4,5], low-density lipoprotein receptor-related protein 4 (Lrp4) [6,7] or agrin [8,9].