Patients with paroxysmal nocturnal hemoglobinuria (PNH) have one or a few clones of mutant hematopoietic stem cells defective in glycosylphosphatidylinositol (GPI) synthesis as a result of somatic mutation in the X-linked gene PIG-A. The mutant stem cell clone dominates hematopoiesis by a mechanism that is unclear. To test whether a lack of multiple GPI-anchored proteins results in dysregulation and expansion of stem cells, we generated mice in which GPI-anchor negative cells are present only in the hematopoietic system. We transplanted lethally irradiated mice with female fetal liver cells bearing one allele of the Piga gene disrupted by conditional gene targeting. Because of the X-chromosome inactivation, a significant fraction of the hematopoietic stem cells in fetal livers was GPI-anchor negative. In the transplanted mice, cells of all hematopoietic lineages contained GPI-anchor negative cells. The percentage of GPI-anchor negative cells was much higher in T lymphocytes including immature thymocytes than in other cell types, suggesting a regulatory role for GPI-anchored proteins at an early stage of T-lymphocyte development. However, the proportions of GPI-anchor negative cells in various blood cell lineages were stable over a period of 42 weeks, indicating thatPiga mutation alone does not account for the dominance of the mutant stem cells and that other phenotypic changes are involved in pathogenesis of PNH.