CD55 and paroxysmal nocturnal hemoglobinuria: Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal hematopoietic disorder caused by a somatic phosphatidylinositol glycan class A mutation, leading to a deficiency of glycosylphosphatidylinositol (GPI)-anchored proteins, such as CD55 and CD59 [1]. Epidemiologic surveys report an incidence of 0.08-0.57 per 100,000 person-years and a prevalence of 38 per million [1].