Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematopoietic stem cell disorder characterized by complement-mediated hemolysis. Abnormal hematopoietic cells from patients with PNH are deficient in glycosylphosphatidylinositol (GPI)-anchored proteins and clonally dominate various hematopoietic lineages in the bone marrow and the peripheral blood. Analysis of many patients with PNH has showed that somatic mutation in the X-linked gene PIG-A is responsible for the GPI- anchor deficiency in PNH. The PIG-A mutation must also be relevant to the clonal dominance of GPI-anchor deficient (GPI-) blood cells because two or more PIG-A mutant clones become dominant in many patients. However, whether the PIG-A mutation alone is sufficient for clonal dominance is not known. To address this question, we generated chimeric mice using Pig-a (the murine homologue of PIG-A) disrupted embryonic stem (ES) cells, in which the animals are chimeric with respect to the surface expression of GPI-anchored proteins. The chimerism of hematopoietic and nonhematopoietic tissues in such mice was always low, suggesting that the higher contribution of Pig-a disrupted GPI- cells had a lethal effect on the chimera. GPI- cells appeared in the peripheral blood of some of the chimeric mice. However, the percentage of GPI- erythrocytes did not increase for 10 months after birth, implying that the Pig-a mutation alone does not immediately cause the clonal dominance of GPI- blood cells; another pathologic or physiologic change(s) in the hematopoietic environments or in the clone itself may be necessary.