Here we show that approximately 50% of known triple seronegative MG patients (i.e., no detectable AChR, MuSK or LRP4 antibodies, AChR-/MuSK-/LRP4-) have serum antibodies against agrin, representing approximately 2–3% of all MG patients in our study. The gene discussed is AGRN; the disease is myasthenia gravis.