The level of expression of IGF-2 receptors positively correlates with repopulating HSC activity. (A) All repopulating HSCs in the total population of fetal liver cells bind IGF-2–hFc. Of the 36.2% of total CD45.2 fetal liver cells that bind the IGF-2–hFc fusion protein (Figure 3Bv), the top 30% and the lower 70% were arbitrarily sorted into the ++ and + fractions, respectively. Similarly, of the 63.8% that cannot bind the IGF-2–hFc fusion protein, the top 70% and the lower 30% were arbitrarily sorted into the - and - - fractions, respectively. As in Figure 3C, reanalysis of sorted cells indicted that the purity of IGF-2–hFc⁺ and IGF-2–hFc^- cells was higher than 80% (not shown). From each group of total fetal liver cells, 10⁴ cells were transplanted together with 10⁵ CD45.1 competitor cells into lethally irradiated CD45.1 mice (n = 4 to 5). Peripheral blood cells were analyzed for the presence of CD45.2⁺ cells in lymphoid and myeloid compartments at 3 weeks and 6 months after transplantation. (B) All repopulating HSCs in the Lin^-Sca-1⁺ subpopulation of fetal liver cells bind IGF-2–hFc. Fifty CD45.2 fetal liver Lin^-Sca-1⁺IGF-2–hFc⁺ and 100 Lin^-Sca-1⁺IGF-2–hFc^- cells sorted as depicted in Figure 3Bv were transplanted together with 2 × 10⁵ CD45.1 competitor cells into lethally irradiated CD45.1 mice (n = 4 to 5). Peripheral blood cells were analyzed for the presence of CD45.2⁺ cells in lymphoid and myeloid compartments at 6 months after transplantation. Error bars indicate SEM.