Reduction in WT HSC activity after transplantation into BMP4-deficient hosts. (A) Primary and secondary transplantation scheme. (B) Analysis of wild-type donor cell engraftment in WT (n = 6) and BMP4 (n = 6) hypomorphic primary recipients 24 weeks after transplantation. (C) Analysis of donor cell engraftment in secondary hosts 20 weeks after transplantation. Wild-type cells that engrafted into primary BMP4 hypomorphic hosts reconstituted long-term hematopoiesis in secondary recipients (n = 12) significantly less well than wild-type cells that engrafted into WT primary hosts before secondary transplantation (n = 9 secondary recipients). *P = .01. In panels B and C, 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.