CD55 and paroxysmal nocturnal hemoglobinuria: Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired blood disorder caused by a somatic mutation in the hematopoietic stem cells of the gene encoding for phosphatidylinositol N-acetylglucosaminyltransferase subunit A (PIGA), causing the defective expression of glycosylphosphatidylinositol-anchored proteins (GPI-APs), including the complement regulatory proteins CD55 and CD59.