Rare chronic diseases, such as paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome (aHUS), generalized myasthenia gravis (gMG), and anti-aquaporin-4 antibody-positive (AQP4-Ab+) neuromyelitis optica spectrum disorder (NMOSD) are caused by uncontrolled complement activation [1–4]. The gene discussed is AQP4; the disease is paroxysmal nocturnal hemoglobinuria.