PNH (paroxysmal nocturnal hemoglobinuria) is a very rare blood disease that causes red blood cells to break apart. PNH occurs because of a genetic change to some of your blood-forming stem cells. These defective stem cells become PNH blood cells that are missing an important coating (protein) that protects them from your immune system. If you have PNH, there will be times when your immune system destroys many red blood cells very quickly, causing low red blood cell counts, as well as blood clots, muscle spasms and dark urine. This destruction of blood cells is called hemolysis.