cell homing efficiency to the spleen was calculated by dividing the absolute number of CD45.1+ KTLS stem cells in the recipient’s spleen by the number of injected KTLS stem cells. To determine the mobilization of recipient’s residual marrow stem cells from marrow, we measured the recipient-origin stem cells (CD45.2+ KLS cells) in the marrow by flow cytometry as described above. Measurement of hematopoietic stem/progenitor cell mobilization after prolonged administration of AMD3100 Transplanted mice that were given AMD3100 at 5 mg/kg body weight or PBS subcutaneously every other day beginning at day +2 for 60 days were used in this study. Additionally, na¨ıve mice that were not transplanted and had not received AMD3100 prior to the study were used as positive control. The mice were given AMD3100 at 5 mg/kg body weight or PBS. Blood samples were collected immediately prior to the injection and 4 hours after injection, and colony-forming units and white blood cell counts determined. Colony-forming units were represented as colony numbers/30 ml blood. In vivo cell division analysis KTLS cells obtained from donor CD45.1 mice were labeled with CFSE [5-(and -6)-carboxyfluorescein diacetate succinimidyl ester, Molecular Probes, Eugene, OR] at a final concentration of 5 mM and incubated at 37uC for 10 min[53]. After the incubation period, the cells were immediately washed three times in cold RPMI/10% FCS. The CFSE-labeled KTLS cells were re-sorted by flow cytometry to obtain a pure population of cells that were CFSE+ c-Kit+ Thy1low Lin2 Sca-1+. The CFSE-labeled KTLS cells were then injected into 10.5 Gy irradiated CD45.2 recipient mice (7,000 KTLS cells/mouse). Beginning at day +2, the recipient mice were injected subcutaneously with AMD3100 at 5 mg/kg or PBS control every other day. The mice were sacrificed at day +7, and CFSE positive cells were measured by flow cytometry. CD45.1+ cells were analyzed for CSFE content. 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