Inducible activation of HOXB4 in vivo. (A) Retroviral vector used in this study. ERT indicates mutated ligand binding domain of murine estrogen receptor-α. (B) Schematic representation of competitive repopulation assay used to assess relative engraftment potential of gene-modified HSC populations in the presence or absence of active HOXB4 as induced by tamoxifen citrate treatment. (C) Competitive repopulation of transduced BM as measured by the percentage of fluorescent-positive cells in peripheral blood. At the indicated time points after transplantation, the contribution of gene modified cells to the production of peripheral blood leukocytes was analyzed by flow analysis. The mean percentage of WT + SF-HOXB4-ERT-IG–transduced cells in the peripheral blood of transplanted recipients is shown plus or minus SEM. ■ represents recipients of 100 μg/mL tamoxifen citrate in drinking water; and □, nonrecipients of tamoxifen citrate. **P < .01, Student t test; n = 5 mice per group.