Cell autonomous requirements for GABPα in maintaining HSCs. (A) Schematic showing the experimental design for induced GABPα inactivation in preestablished BM chimeras. The GABPα-floxed animals were previously generated in 129/SvEv embryonic stem cells and were crossed to C57BL/6 strains (WT or Mx1Cre transgenic) for 4 generations. To avoid potential rejection of BM grafts, the F1 progeny of B6.SJL and 129/SvEv crossing was used as hosts for generation of BM chimeras. The F1 hosts expressed both CD45.1 and CD45.2 congenic markers, whereas the donor-derived cells were positive for CD45.2 only, allowing direct distinguishing of the cell origins in the BM chimeras. (B) BM cells from Mx1Cre-GABPα^FL/+ or Mx1Cre- GABPα^FL/− were injected into irradiated F1 progeny to establish BM chimeras, and the LSK cells were largely of donor origin (CD45.2⁺ > 95%, top panels). The BM chimeras were treated with pIpC as in Figure 1A, and 9 days after the last pIpC injection, the LSK population and its origin were analyzed (bottom panels). Note that LSKs from pIpC-treated Mx1Cre-GABPα^FL/− BM chimeras were significantly diminished, and a substantial portion of the remaining LSKs was of host origin. (C) Experimental design for induced GABPα inactivation in the presence of WT reference cells. (D) BM cells from Mx1Cre-GABPα^FL/+ or Mx1Cre-GABPα^FL/− were mixed with those from B6.SJL at 1:1 LSK ratio and injected into irradiated F1 progeny to establish mixed BM chimeras (top panels). The hosts were treated with pIpC as in panel B and analyzed for LSK frequency and origin. All data are representative of 3 independent experiments with similar results.