Paroxysmal Nocturnal Hemoglobinuria (PNH) is a disorder in which the most immature of the blood forming cells in the bone marrow, the stem cells, are lacking proteins that prevent them from being destroyed by the body. The destruction is intermittent and usually occurs at night. Because the abnormal stem cells form all of the blood's cells , there will be problems in the platelets (responsible for clotting), red cells (leading to anemia) and white cells (important in fighting infections) predisposing them to dysplastic changes including transformation into leukemia.