Differences in STAT5 activation by FLT3-ITD and ligand-stimulated Flt3WT. STAT5 (A) or FLT3 (B) activation was examined by immunoblot analysis of Ba/F3 cell lysates starved of IL-3 for 4 hours. For FL stimulation, starved cells were treated with 100 ng/mL FL and incubated for 2, 5, 10, or 20 minutes at 37°C. For stimulation with IL-3, starved cells were treated with 10 ng/mL IL-3 and incubated for 10 minutes at 37°C Total cell lysates were immunoprecipitated with an anti-STAT5 antibody or anti-FLT3 antibody and separated by SDS-PAGE. Immunoblot analysis was conducted with an antiphosphotyrosine (4G10) antibody (top rows). The blots were then stripped and reprobed with anti-STAT5 or anti-FLT3 antibody (bottom rows). The same lysates were also analyzed for p44/42 MAP kinase (Erk) activation (C) by immunoblotting with an anti–phospho-Erk antibody (top row) and anti-Erk antibody (bottom row).