MUSK and myasthenia gravis: Anti-acetylcholine receptor (AChR) (3) antibody, anti-muscle-specific receptor tyrosine kinase (MuSK) (4) antibody, and anti-low-density lipoprotein receptor-related protein 4 (LRP4) (5) antibody have been shown to be associated with MG, with reported positivity rates of approximately 85% for AChR, 1–10% for MuSK, and 1–5% for LRP4.