Conditional deletion of STAT5 in adult host HSCs, but not stroma, permits efficient stem cell replacement. (A) The percentage of deletion of STAT5 in Gr-1⁺ cells 7 days (D7) or 4 months (4M) after pI:pC treatment (3 doses, 16 mg/kg, every other day) was determined by real-time quantitative PCR (n = 5) normalized to wild-type STAT5 (■). The control is STAT5ab^flox/null mice, which should yield 50% the amount of wild-type STAT5. Deletion in the KLS pool on day 7 is also shown (□). (B) Percentage of CD45.1-derived overall and Gr-1, B220, Ter119, or CD4 cells in the peripheral blood of each recipient 22 to 40 weeks after transplantation into wild-type (n = 7) or STAT5 KO (n = 10) from 2 separate injection dates. (C) Percentage of CD45.1-derived Gr-1, B220, or c-Kit⁺Lin⁻Sca-1⁺ cells in BM from each recipient (1-5 in Figure 4B) 40 weeks after transplantation. (D) A representative dot plot from each secondary recipient mouse (nos. 1-5) 16 weeks after transplantation with one primary donor equivalent per 5 secondary recipients. Each engrafted mouse in panel C was transplanted into 3 lethally irradiated CD45.2 recipient, and the numbers shown are mean plus or minus SD values of the percentage of donor chimerism from all 3 recipients. (E) Experimental outline, representative fluorescence-activated cell sorting, and average donor chimerism (mean ± SD) from chimeric mice before and after transplantation with 5 × 10⁶ GFP-transgenic BM cells.