Ets-1 interacted physically with Stat1 after JAK2 activation in vivo. Expression plasmids of HA-tagged Ets-1 or Flag-tagged Stat1 was transfected into Cos7 cells with or without expression vector encoding the TEL-JAK2 protein. Lysates from transfected cells were immunoprecipitated (IP) with anti-HA Ab (A) or anti-Flag Ab (B-C). Total protein (30 μg) from each transfected sample was loaded as an input. Immunoblots (IB) were developed with anti-HA Ab (revealing Ets-1) or anti-Flag Ab (revealing Stat1) as indicated. (D) Lysates from CD43⁻–purified B splenocytes activated for 1 hour with 25 μg/mL of LPS and 100 ng/mL of IFN-γ or left in medium alone (Med) were immunoprecipitated (IP) with anti–Ets-1 Abs. Total protein (30 μg) from each sample was loaded as input controls. Immunoblots (IB) were developed with anti–Ets-1 or anti-Stat1 Abs as indicated.