Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal disorder of hematopoietic stem cells (HSCs) that is caused by somatic mutation of the phosphatidylinositol glycan A (PIG-A) gene, which is located on chromosome Xp22.1. The gene discussed is PIGA; the disease is paroxysmal nocturnal hemoglobinuria.