PIG-Amutations in health and disease.PIG-A mutations in PNH (upper panel) arise from a multipotent hematopoietic stem cell (HSC). Expansion and differentiation of the PIG-A mutant HSC results in clinical disease. In healthy control subjects, most, if not all, PIG-A arise from colony-forming cells (CFC). CFC can differentiate, but have no self-renewal capacity; hence, these PIG-A mutations do not result in disease.