CD55 and paroxysmal nocturnal hemoglobinuria: Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematopoietic stem cell disorder with a triad of chronic hemolysis, bone marrow failure, and thromboses.1 Mutation involving Phosphatidylinositol glycan (PIG-A) gene causes failure to synthesize glycophosphatidylinositol anchor proteins, including the complement regulators CD55 and CD59, on the surface of affected blood cells, making them vulnerable to complement activation, the formation of membrane attack complex, and eventually hemolysis.2