Dose dependence of GM-CSF–induced tyrosine phosphorylation of JAK2. Quiescent ECs were incubated with different amounts of GM-CSF (ng/mL) in M199-BSA for 5 minutes at 37°C, lysed, and proteins immunoprecipitated with anti-JAK2 antibody. Immunoprecipitate was analyzed by SDS-PAGE followed by immunoblotting with antiphosphotyrosine antibody. Subsequently, blots were re-probed with anti-JAK2 antibody (lower panel). This experiment is representation of five performed with similar antibody. In this experiment, the 200-kD molecular marker remained at the top of the gel.