Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematopoietic stem cell (HSC) disorder characterized by the clonal expansion of a PIG-A mutated stem cell and consequent defective synthesis of glycosil phosphatidyl-inositol-anchored proteins, complement-mediated hemolysis, increased incidence of thrombosis, and bone marrow failure. The gene discussed is PIGA; the disease is paroxysmal nocturnal hemoglobinuria.