G-CSF–induced tyrosine phosphorylation of Stat1 is 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 (G) or IFN-γ (I), were immunoprecipitated with antibodies against Stat1. Precipitated proteins were separated by SDS-PAGE, transferred to nitrocellulose, and probed with antibodies to phosphotyrosine (α-PTyr). The filters were stripped and reprobed with a Stat1 polyclonal antibody. The cells examined included the γ2A cells expressing the G-CSF receptor (GR), the receptor with wild-type Jak2 (GR/J2), or the receptor with a kinase-inactive Jak2 mutant (GR/J2KE).