MUSK and myasthenia gravis: In 15% of patients, anti-AChR antibodies are not detected; of those, ~ 50%, comprising 5–8% of all AChR-negative MG cases, are positive for antibodies against muscle-specific kinase (MuSK), a transmembrane polypeptide expressed at the neuromuscular junction that plays a fundamental role in AChR clustering [36].