PIGA and paroxysmal nocturnal hemoglobinuria: By its nature, PNH is a clonal disorder affecting either one or several pluripotent hematopoietic stem cells of the bone marrow harboring a genetically acquired somatic loss of function mutations in the phosphatidylinositol N-acetylglucosaminyltransferase subunit A (PIG-A) which is encoded by the PIGA gene resulting in a defective glycosylphosphatidylinositol (GPI)-anchored protein synthesis with a reduced expression or even absence of GPI-anchored proteins, in particular CD55 and CD59.