G-CSF–induced tyrosine phosphorylation of G-CSF receptors and Stats 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 G-CSF receptor antibody, Stat3 antiserum, or Stat5 antiserum, respectively. 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 G-CSF receptor polyclonal antibody, Stat3 antiserum, or Stat5 antiserum.