Loss of Stat5 results in increased levels of apoptosis in fetal liver cells. (A) Freshly isolated fetal livers from E13.5 to E15.5 were stained for Ter-119 and annexin V to determine rates of apoptosis (data are presented as means ± SD; n = 3) for each genotype and time point. (B) CFU-E colonies derived from wt or Stat5^−/− fetal liver cells using the indicated Epo concentrations (data are presented as means ± SD; n = 4). (C) E13.5 fetal liver cells of wt and Stat5^−/− embryos were infected with a retrovirus encoding GFP. TfR-1^high/c-Kit⁺/GFP⁺ cells were isolated by FACS after 7 days under self-renewal conditions (cytospin; right panel). (D) A total of 1.5 × 10⁷ TfR-1^high/c-Kit⁺/GFP⁺ cells were injected into the tail vein of lethally irradiated mice (950 rad) and Ter-119⁺/GFP⁺ bone marrow cells were scored 3 days later (mean ± SD; n = 4). *P < .05; **P < .01.