Paroxysmal nocturnal hemoglobinuria (PNH) is a rare disease in which red blood cells break down earlier than normal. Persons with this disease have blood cells that are missing a gene called PIG-A. This gene allows a substance called glycosyl-phosphatidylinositol (GPI) to help certain proteins stick to cells. Without PIG-A, important proteins cannot connect to the cell surface and protect the cell from substances in the blood called complement. As a result, red blood cells break down too early. The red cells leak hemoglobin into the blood, which can pass into the urine. This can happen at any time, but is more likely to occur during the night or early morning. The disease can affect people of any age. It may be associated with aplastic anemia, myelodysplastic syndrome, or acute myelogenous leukemia. Risk factors, except for prior aplastic anemia, are not known. The outcome varies. Most people survive for more than 10 years after their diagnosis. Death can result from complications such as blood clot formation (thrombosis) or bleeding. In rare cases, the abnormal cells may decrease over time.