GM-CSF–induced tyrosine phosphorylation of JAK2 associated with GM-CSFR β chain. Quiescent ECs were incubated for 5 minutes at 37°C in M199-BSA with GM-CSF (50 μg/mL) and cell lysate were immunoprecipitated with β-chain antiserum. After protein solubilization from Protein A-Sepharose, samples were further subjected to a second immunoprecipitation with anti-JAK antibody. Samples were analyzed by SDS-PAGE followed by immunoblotting with antiphosphotyrosine antibody. Subsequently, blot was reprobed with anti-JAK2 antibody (lower panel). The figure is representative of five similar experiments.