A, SUM44 cells were cultured for 6 days, treated with different doses of GW572016 for 1 h, and then with or without heregulin (10 ng/ml) for 15 minutes. The cells were lysed and the lysates were immunoprecipated with anti-HER2, anti-HER3, or anti-HER4 antibody and blotted with anti-phosphotyrosine antibody. B, SUM44 cells were grown with or without heregulin (10 ng/ml) in the presence or absence of GW572016 (1 μM) for 6 days, at which time the number of cells were counted. Mean ± SD of triplicates are shown, representative of 3 experiments. ***, P < 0.001 versus control.