The most common subtype of MG is the production of autoantibodies directed against the nicotinic acetylcholine receptor (AChR) and is controlled primarily by the IgG1 and IgG3 subclasses.318,319 In addition to this, autoantibodies against muscle-specific tyrosine kinase (MuSK) are also present in many patients.320 Unlike AChR MG, MuSK MG isoforms are most often associated with IgG4 subclass autoantibodies.321,322 The level of Fab glycosylation of IgG4 antibodies was significantly elevated in MuSK MG, but this change was not found in the IgG subclass of AChR MG. Here, MUSK is linked to myasthenia gravis.