WT HSC activity is poorly supported in a BMP4-deficient microenvironment. (A) Approximately 10 months after separation, equal numbers of donor cells were FACS sorted from 3 pairs of mutant/WT parabionts and cotransplanted into secondary recipients along with an equal number of carrier BM cells. (B) Analysis of donor cell engraftment for the experiment diagrammed in panel A, 16 weeks after transplantation. WT cells conditioned in BMP4 hosts were drastically outcompeted by mutant cells conditioned in WT hosts. Two of a total of 5 secondary recipients were engrafted with WT cells. *P < .002. (C) To test host cell activity, equal numbers of host cells were also FACS sorted from 2 mutant/WT parabiont pairs approximately 10 months after separation and cotransplanted into secondary recipients along with an equal number of carrier BM. (D) Analysis of donor cell engraftment for the experiment diagrammed in panel C, 16 weeks after transplantation. Significant differences between WT and mutant cell engraftment were not detected, indicating comparable host cell activity. Four secondary recipients were assayed. #P = .12. In panels B and D, total donor cells are shown as a percentage of live cells. Individual lineages are shown as the percentage of donor-derived cells. Error bars show SEM.