Competitive repopulation in runx1 mutants reveals a high stem cell frequency. (A) Experimental schema: adult ubi:GFP and ubi:mCherry donor marrow was harvested and transplanted at different ratios into runx1-mutant embryos at 2 dpf. Donor-derived GFP+ and mCherry+ chimerism was assessed 2 months posttransplant. (B) Percentage of transplanted runx1 mutants engrafted with GFP+ donor cells after transplantation with varying proportions of mixed GFP and mCherry cells. Engraftment was defined as ≥5% GFP+ myeloid cells. (C) Stem cell frequency was calculated by using the Extreme Limiting Dilution Analysis calculator.27 The tested subjects included all transplanted runx1-mutant larvae, and subjects with a response were fish that met the 5% myeloid chimerism threshold.