SrfKO^H fetal liver cells fail to engraft durably. 10⁶ total CD45.2⁺Srf⁺ or SrfKO^H fetal liver cells were used to reconstitute wild-type CD45.1⁺ animals after high-dose (2 × 6 Gy) irradiation as indicated. Donor Srf⁺ and SrfKO^H cells were distinguished by CD45.2 marker or use of the mT/mG system (mT, Srf⁺; mG, SrfKO^H; see supplemental Figure 3). Solid symbols, Srf⁺; open symbols, SrfKO^H. For raw data and results after low-dose irradiation (2 × 4.5 Gy), see supplemental Figure 4. (A) Peripheral blood analysis. Left, total donor and host cells were distinguished using CD45.2 (donor) and CD45.1 (host); middle and right, donor cells were analyzed for contribution of Srf⁺ (mT) or SrfKO^H (mG) cells to B220 (B cell) or Gr-1 (granulocyte) lineages by gating on mT or mG as appropriate. Mean values are shown (n = 4-6 per condition). (B) Srf⁺ (mT) and SrfKO^H (mG) cells in reconstituted bone marrow (left) and spleen (right) at 18 weeks. (C) Srf⁺ (mT) and SrfKO^H (mG) LSK cells in reconstituted bone marrow at 18 weeks. Left, LSK-gated bone marrow; right, proportion of CD150⁺ cells. (D) Secondary transplants of bone marrow cells from animals reconstituted for 18 weeks with Srf⁺ or SrfKO^H fetal liver cells. Reconstitutions were performed using 2 × 10⁵Srf⁺ (mT) or SrfKO^H (mG) bone marrow cells; irradiation was at 2 × 6 Gy. Analysis was done as in (A). See also supplemental Figure 4C. (E) Proportion of LSK cells in bone marrow 16 weeks after the secondary transplant in (C). (F) SrfKO^H cells compete ineffectively with Srf⁺ cells for bone marrow engraftment. Donor Srf⁺ and SrfKO^H cells were engrafted either alone (left and center panels) or in 1:19 Srf⁺:SrfKO^H ratio (right panel) after 2 × 6 Gy irradiation, and the proportions of LSK-gated Srf⁺ (mT), SrfKO^H (mG), and host cells were measured 12 weeks later.