Patients with paroxysmal nocturnal hemoglobinuria who had a poor response to eculizumab therapy were found to have a genetic polymorphism in C5 that prevents binding by the antibody. Paroxysmal nocturnal hemoglobinuria (PNH) arises as a consequence of clonal expansion of hematopoietic stem cells that have acquired a somatic mutation in the gene encoding phosphatidylinositol glycan anchor biosynthesis class A ( PIGA ). 1 – 3 The resulting hematopoietic cells are deficient in glycosylphosphatidylinositol-anchored proteins, including the complement regulatory proteins CD55 and CD59; this accounts for the intravascular hemolysis that is the primary clinical manifestation of PNH. 4 – 6 PNH frequently develops in association with disorders involving bone marrow failure, particularly aplastic anemia. Thrombosis is a major cause of PNH-associated morbidity and mortality, particularly among white patients. 7 – 9 Eculizumab (Soliris, Alexion Pharmaceuticals) . . .