AQP4 and paroxysmal nocturnal hemoglobinuria: Eculizumab is a humanized mAb that is directed against C5 and which has been transformative for patients with paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome (aHUS), refractory generalized myasthenia gravis (gMG) and aquaporin-4-IgG-positive neuromyelitis optica spectrum disorder [1–5].