Inhibition of tyrosine phosphorylation of STAT3 by dominant negative STAT3. (A) WEHI-3B D⁺ cells expressing the chimeric LIFR (W238) and ▵STAT3 or the chimeric LIFR alone (Pur^rNeo^r) were stimulated with hGM-CSF (10 ng/mL) or IL-6 (100 ng/mL) for 10 minutes. STAT3 was immunoprecipitated from cell lysates using anti-STAT3 antibodies (C20; Santa Cruz Biotechnology), and then probed with either antiphosphotyrosine antibodies or anti-STAT3 antibodies (Transduction Laboratories). (B) Expression of ▵STAT3 in WEHI-3B D⁺ cells. STAT3 was immunoprecipitated from cell lysates using anti-STAT3 antibodies (K-15; Santa Cruz Biotechnology) and then probed with anti-STAT3 antibodies (Transduction Laboratories).