G-CSF–induced tyrosine phosphorylation of Jaks was suppressed by a kinase-dead Jak2. Whole cell extracts from the indicated cells, with or without treatment for 5 minutes at 37°C with G-CSF, were immunoprecipitated with antisera against Jak1 or Jak2. Precipitated proteins were separated by SDS-PAGE, transferred to nitrocellulose, and probed with antibodies to phosphotyrosine (α-PTyr). Subsequently, filters were stripped and reprobed with Jak1 or Jak2 polyclonal antibodies.