Epcr⁺ wt BM cells have a repopulation advantage in wt mice. BM cells were isolated by fluorescence-activated cell sorting as described in “Methods.” After sorting, a number of 1.5 × 10⁴ Epcr⁺lin⁻ or 1.5 × 10⁶ Epcr⁻lin⁻ cells were transplanted per recipient mouse. (A) LSK cell number in the BM and spleens of wt mice transplanted with Epcr⁺ or Epcr⁻ lin⁻ BM cells 1 week after transplantation. (B) Total LSK cell number (white bar) and the respective donor contribution (gray or blue bar) in the BM and spleens of wt mice transplanted with Epcr⁺ or Epcr⁻ lin⁻ BM cells 4 weeks after transplantation. (A-B) n = 5; mean plus or minus SD; 1-way-ANOVA with Dunnett multiple comparisons test [##P ≤ .01]) (C) Donor contribution in the PB over the course of the experiment from 1 to 20 weeks after transplantation. In the Epcr⁺ group, donor contribution increased significantly (mean plus or minus SD; 2-tailed, unpaired Student t test with Welch correction to compare Epcr⁺ vs Epcr⁻ at each time point [**P ≤ .01]). (D) Total LSK cell number (white bar) and the respective donor contribution (gray or blue bar) in the BM of wt mice transplanted with Epcr⁺ or Epcr⁻ lin⁻ BM cells 20 weeks after transplantation. (Epcr⁺, n = 9; Epcr⁻, n = 6; mean plus or minus SD; 1-way-ANOVA with Dunnett multiple comparisons test [##P ≤ .01]). (E) Donor contribution on LT-HSCs in the BM 20 weeks after transplantation (Epcr⁺, n = 9; Epcr⁻, n = 6; mean plus or minus SD; 2-tailed, unpaired Student t test with Welch correction [**P ≤ .01]). (F) Flow cytometric cell-cycle analysis of naive wt and Mpl^−/− LSK cells and LT-HSCs (LSK, CD150⁺, CD48⁻) discriminated by the expression of Epcr. Mpl^−/− LSK cells that are negative for Epcr are significantly more in G1 compared with Epcr⁻ wild-type LSK. (C57Bl/6J n = 4, Mpl^−/− n = 3, mean plus or minus SD, 2-way ANOVA with Tukey multiple comparisons test [##P ≤ .01]). (G) Epcr⁺ and Epcr⁻ lin⁻ populations were sorted independently from 3 mice for subsequent RNA isolation and cDNA synthesis. The expression of the 2 anti-apoptotic genes Bcl-2 and Bcl-xL, and the 2 cell-cycle inhibitors Cdkn1a/p21 and Cdkn1c/p57 was measured by real-time qPCR (n = 3; mean plus or minus SD).