Stat3 activation and apoptotic erythroblasts in ΔHmga2/JAK2^V617Fmice. (A) Myeloid colonies from 1 × 10⁴ BM MNCs (N = 3 for each genotype, means ± SEM). *P < .05; **P < .01; ***P < .001 (Tukey-HSD tests). (B) Numbers of CFU-E colonies (means ± SEM) from 1 × 10⁵ of BM MNCs (N = 3 for each genotype). *P < .05; **P < .01; ***P < .001 (Tukey-HSD tests). The P values calculated by ANOVA were <.01 for both EPO concentrations and mouse transgenes. (C) Western blotting of Stat3 and pStat3 in BM MNCs without cytokine stimulation in each tg mouse. Left panel, A representative result; right panel, mean pSTAT3/STAT3 concentration (from 2 independent experiments) relative to WT. (D) Stat3 mRNA in BM MNCs by qRT-PCR (means ± SEM; N = 3 for each). *P < .05 (Tukey-HSD test). (E) FACS for pStat3 in BM MNCs incubated in the absence (top) and presence (bottom) of IL-3. The horizontal and vertical axes, respectively, indicate the brightness of pStat3 and the cell counts. (F) FACS for apoptosis of CD71⁺Ter119⁺ erythroblasts by Annexin V/7-Aminoactinomycin D (7AAD) staining. Top panel, A representative result; bottom panel, mean ratio of Annexin V⁺/7AAD⁻ apoptotic erythroblasts from 3 independent experiments. *P < .05; **P < .01; ***P < .001 (Tukey-HSD tests). (A-F) H, ΔHmga2; J, JAK2^V617F; HJ, ΔHmga2/JAK2^V617F.