Expression of ZAP-70 and its association with the FcεRIγ subunit in THP-1 cells. Lysates from unstimulated (−) or FcγRI-stimulated (+) THP-1/LN or THP-1/ZAP-70 cells were immunoprecipitated with polyclonal rabbit anti-Syk (a and d), anti–ZAP-70 (b and f ), or anti-FcεRIγ (c, e, and g) antibodies and separated on 7.5% polyacrylamide gels. Blots were probed with either a monoclonal antiphosphotyrosine (a, b, and c), polyclonal rabbit anti-Syk (d and e), or monoclonal anti–ZAP-70 (f and g) antibody. The position of the 67-kD molecular weight marker is indicated in each panel (−). These data are representative of results obtained in five independent experiments.