Hematopoietic stress confers a competitive advantage to p57-KO HSCs. (A) Schematic representation of competitive repopulation analysis. CD45.1⁺ control FLMCs were mixed 1:1 with a test population of either p57-WT or p57-KO FLMCs (EGFP^+/+), and then transplanted into lethally irradiated recipient mice (n = 10). Stably engrafted mice with p57-WT or p57-KO hematopoiesis were analyzed at steady state (B) or after chemotherapy (C). (B) Multilineage peripheral blood engraftment was analyzed by flow cytometry at the indicated times after reconstitution to assess the proportion of p57-WT or p57-KO cells (n = 10). (C) To study the effect of myelosuppressive stress in competitively transplanted recipient mice, recipients were allowed to reach homeostasis (10 wk), and then given a single dose of myelosuppressive chemotherapy. Multilineage engraftment was assessed as described previously to monitor the relative contribution of p57-WT or p57-KO hematopoiesis during regeneration (n = 10). The percentage of donor cells was normalized to the day 0 measure for each mouse. The orange box represents the standard deviation of the day 0 measurements. (D) Kaplan-Meier survival curve is shown for chimeric mice stably engrafted with p57-WT or p57-KO hematopoiesis (n = 10), and then cyclically treated every 4 wk with a myelosuppressive dose of 5FU. All quantified data are shown as mean ± SEM (*, P < 0.05; **, P < 0.01; ***, P < 0.001, or if undesignated, the comparison was not significant).