MUSK and myasthenia gravis: Since the identification of the first and most abundant MG antibody target, the AChR, in 1976 [9,10], MG-associated Abs directed against other postsynaptic structures, such as muscle-specific kinase (MuSK) [11,12] and lipoprotein-receptor-related protein 4 (LRP4) [13,14] have been described.