MUSK and myasthenia gravis: Additionally, the antibodies in AChR-positive MG belong to the IgG1 and IgG3 subclasses, while most pathogenic antibodies of MuSK-Abs are of the IgG4 subclass, which can neither activate complement nor induce antigenic modulation (8), and only weakly bind Fc receptors expressed on immune cells, masking the site of normal MuSK-LRP4 interaction, thereby impeding AChR aggregation and impairs their alignment in the postsynaptic membrane (7, 9).