Long-term HSC deficiency of Runx site mutants is related to exhaustion and is PU.1 dependent. (A) Reduced capability of PU.1-URE-mRunx mice to regenerate bone marrow after repetitive injuries by weekly administration of 5-FU 150 mg/kg injected intraperitoneally. Results are shown as Kaplan-Meier survival curves (n = 9; P < .0001 [Mantel-Cox]). (B) Serial transplantation assays. Rounds of transplantations with 5 × 10⁵ bone marrow cells were performed at 16-week intervals. Bar graphs indicate survival percentage after each round of transplantation (TX) (n = 5). (C) Whole bone marrow cells (1 × 10⁶) of indicated donor mice (CD45.2⁺) were co-transplanted with equal amounts of CD45.1⁺ WT bone marrow cells into lethally irradiated CD45.1⁺ recipients. Bar graphs show donor chimerism in blood after 6 months (average + SD; n = 5; **P < .01). Loss of chimerism of PU.1-URE-mRunx donors was restored by breeding to a strain harboring a human transgene expressing PU.1 (+hPU.1-TG).