Competitive repopulation of congenic STAT5ab^-/- mouse strains against each other. To generate mice that could be use for a direct head-to-head competitive repopulation assay against c-Mpl^-/- BM cells, a congenic strain of the STAT5ab^-/- mouse was first generated. The C57BL/6 background STAT5ab^-/- mouse was crossed onto the HW80 congenic background. To determine whether the long-term repopulating HSC phenotype of the HW80 background mouse was the same as the original C57BL/6 background, head-to-head competitive repopulation experiments were set up by mixing 1:1 donor BM cells from each mouse strain. Shown are the results of 3 separate experiments in which mice were bled 16, 11, and 16 weeks, respectively, following transplantation and packed red blood cells were analyzed by hemoglobin electrophoresis for the relative contribution of Hb_d and Hb_s. Control samples were run with each experiment to demonstrate the typical bands for Hb_d and Hb_s. For experiment 2, competitions were also set up between STAT5ab^-/- or c-Mpl^-/- BM (Hb_s) and wild-type BM (Hb_d). For experiment 3 wild-type BM grafts (2) on either strain also were competed against each other. Each experiment was initiated from 1 to 2 donor mice and all recipient mice are shown. The %Hb_s for each individual mouse is shown below each lane.