STAT3β impairs colony formation capacity of MLL-AF9–transformed cells. (A) Workflow of FLCs and BMCs harvested from Stat3βTG mice and wt littermates. MLL-AF9–transformed cells were subsequently used for in vitro analysis and transplantation. (B) Western blot analysis showing STAT3 and phosphorylated STAT3 (pSTAT3) expression in FLCs transformed with MLL-AF9. Stat3βTG expression was induced via stimulation with IFNβ. (C) in vitro proliferation was analyzed via growth curves and cell cycle analysis by flow cytometry. Data, indicating 3 independent experiments carried out in triplicates, were compared using the Student t test, and differences were found not to be statistically significant. (D) Methylcellulose-based colony formation assays were analyzed by number of colonies upon 4 replatings (1 replating shown; n = 12 per group). Representative pictures of BMC colonies are shown. Data, indicating 2 independent experiments, were compared using the Student t test. **P < .01, ****P < .0001. NSG, NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ.