Paroxysmal nocturnal hemoglobinuria, or PNH, is a hematopoietic stem cell disorder.

Hematopoietic stem cells are developed in the bone marrow, where they grow and eventually develop into red blood cells, white blood cells and platelets. In patients with PNH, these stem cells are defective causing red blood cells to break down too early causing severe abdominal pain crises, severe headaches, back pain, excessive weakness, fatigue, recurrent infections and blood clots.

Here are three links to learn more:
John Hopkins
NORD
U.S. National Library of Medicine