Time course of GM-CSF–induced tyrosine phosphorylation of GM-CSFR β chain. Quiescent ECs were incubated with GM-CSF (20 ng/mL) in M199-BSA at 4°C and cell lysates were immunoprecipitated with β-chain antiserum. Immunoprecipitate was analyzed by SDS-PAGE, followed by immunoblotting with antiphosphotyrosine antibody and then with anti–β-chain antibody. The figure is representative of two identical experiments.