Stat3 and Stat5 activation by G-CSF. Murine myeloid precursor cells expressing wild-type G-CSF receptor (FDC-P1; DA3) or the endogenous G-CSF receptor (NFS60) were either stimulated (−) or stimulated with G-CSF (G) or IL-3 (I) for 5 minutes. Cell lysate was incubated with antisera against Stat3, Stat5a, or Stat5b and the immunoprecipitates were analyzed by Western blotting with a monoclonal antibody against phosphotyrosine (4G10).