(A) Phosphorylation of JAK2 in mutant cell lines. Cells (10⁷) were incubated in the absence or presence of 10 ng/mL GM-CSF for 15 minutes. Total cellular proteins were immunoprecipitated with anti-JAK2 antibody and subjected to 7.5% SDS-PAGE and immunoblot analysis with antiphosphotyrosine antibody. (B) Phosphorylation of STAT5 in mutant cell lines. Cells (5 × 10⁶) were incubated in the absence or presence of 10 ng/mL GM-CSF for 15 minutes and cellular proteins were subjected to 7.5% SDS-PAGE and immunoblot analysis with anti-phospho-STAT antibody. (C) Phosphorylation of STAT5 in Ba/F3 −GMRαβ F₈ cells in response to two concentrations of GM-CSF.