MUSK and myasthenia gravis: While approximately 5–6% of patients diagnosed with seronegative acetylcholine receptor (AChR) MG have antibodies against MuSK, lipoprotein receptor-related protein 4 (LRP4) has been found in 2–27% of patients with double-seronegative MG (dSnMG) (where both AChR and MuSK antibodies are absent) [5,8,9,10].