MUSK and myasthenia gravis: In myasthenia gravis (MG) associated with anti-MuSK IgG4, FAE enhances the pathogenic function of these autoantibodies by generating monovalent bispecific antibodies unable to cluster MuSK but capable of blocking the interaction between MuSK and Low-density lipoprotein receptor related protein 4 (Lrp4) (15).