Runx1^−/− status leads to stem cell exhaustion. (A) Absolute number of KSL CD34⁻Flt3⁻ cells, Lin⁻ SP cells, KSL CD34⁺Flt3⁺ cells and KSL CD34⁺Flt3⁻ cells per 0.5 million BM cells from Runx1^+/+ and Runx1^−/− mice of 2 distinct ages (10 and 40 weeks old). Each group comprises 3 to 4 mice. (B) Limiting dilution analysis using varying numbers of BM cells from 40-week-old CD45.2⁺Runx1^+/+ (▲), or Runx1^−/− (■) mice. Mice were considered negative when the percent chimerism was less than 1%. Left panel: estimated frequencies of the repopulating cells are indicated as vertical dashed lines (1 repopulating cell per indicated numbers of BM cells) for each genotype. Right panel: for each indicated number of transplanted cells from CD45.2⁺Runx1^+/+ or Runx1^−/− mice, the proportion of mice that are positive for test CD45.2⁺ cells is given as (number of positive mice)/(number of analyzed mice). Frequencies of HSCs were calculated using Poisson statistics. (C) GFP chimerism in PB of recipients of Runx1^+/+ (n = 6) and Runx1^−/− (n = 6) cells at 6 and 40 weeks after transplantation. Each open circle represents data from an individual mouse and each closed circle is the average of a cohort. Statistical difference using unpaired Student t test is given at the bottom. NS indicates not significant. (D) Kaplan-Meier survival curves of secondary recipients of mock MIG vector–transfected Runx1^+/+ (dashed line; n = 10) and Runx1^−/− (solid line; n = 10) BM cells. Circles represent end point of analysis.