KLS cells expressing STAT5aΔN^S711F have reduced survival and bcl-2 expression. (A) STAT5ab^null/null FL cells at E14.5 and E15.5 were transduced with either MSCV/IRES-GFP vector control (IR), STAT5a^S711F, or STAT5aΔN^S711F, followed by transplantation into lethally irradiated recipient mice. Fifteen days later, apoptosis was assayed on the GFP⁺c-Kit⁺Lin⁻Sca-1⁺ (GFP⁺KLS) fraction of BM cells from each group by annexin V–DAPI assay. Early apoptotic cells were defined as annexin V⁺ DAPI⁻. A representative flow cytometric plot shows annexin V and DAPI gating on GFP⁺ KLS cells among 3 groups. (B) Percentage of annexin V⁺ DAPI^– GFP⁺ KLS cells from 3 independent mice among each group (STAT5a^S711F vs STAT5aΔN^S711F, P < .01; Student t test). Error bars represent SD of the mean. (C) A representative histogram showing bcl-2 expression in GFP⁺KLS cells. (D) Bcl-2 expression was determined by mean fluorescence intensity minus IgG background after intracellular staining (STAT5a^S711F: n = 4, STAT5a^S711FΔN: n = 3; STAT5a^S711F vs STAT5aΔN^S711F, P < .01; Student t test). Error bars represent SD of the mean.