Reduced colony-forming potential of Stat5a/b-null GMPs. (A) Complete deletion of Stat5a/b in all GMPs from Stat5^f/f:Mx1-Cre mice after PolyIC administration. No outgrowth of Stat5^f/f cells after 6 days of culture. #1, #2, and #3 indicate independent experiments with cells from Stat5^f/f:Mx1-Cre; #4, Stat5^f/f. (B) Phosphoflow cytometry of GMPs revealed STAT5 activation by GM-CSF, and no aberrant STAT3 activation in GMPs from Stat5^f/f:Mx1-Cre. One representative experiment of 2 is shown. (C-E) Colony formation assay with GMPs from Stat5^f/f and Stat5^f/f:Mx1-Cre mice cultured with either G-CSF or GM-CSF. Scale bars represent 100 μm. (D) The number of colonies from Stat5a/b-null GMPs cultured in GM-CSF was reduced. No difference in colony formation was observed with G-CSF. Data are mean ± SD; n = 3 independent experiments *P = .03. (E) Colonies derived from Stat5a/b-null GMPs cultured in GM-CSF were significantly smaller, consisting of much fewer cells; no difference was seen in G-CSF–derived colonies. Data are mean ± SEM. For G-CSF n = 40 colonies of each group from 2 experiments, for GM-CSF n = 45 colonies of Stat5^f/f, for Stat5^f/f:Mx1-Cre n = 37 colonies from 2 experiments. **P < .001.