Time course of GM-CSF–induced tyrosine phosphorylation of JAK1. Quiescent ECs were incubated with 10 ng/mL of GM-CSF in M199-BSA 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 reprobed with anti-JAK1 antibody (lower panel). This experiment is representative of two performed with similar results.