ErbB antagonists inhibit growth factor-induced proliferation and signalling. (a) The anti-proliferative effect of the ErbB antagonists was confirmed in BT474 and MDAMB468 cells, using Ki67 staining as a proliferation marker (48 hours post drug treatment). Data presented as mean ± standard error of the mean from at least three experiments. (b) Cells were serum-starved for 4 hours prior to the addition of vehicle only (con), Trastuzumab (100 μg/ml), Lapatinib (100 nM) or Gefitinib (10 μM) for 24 hours. Cells were stimulated with epidermal growth factor (EGF) (100 ng/ml) for 15 minutes at 37°C. Activation of Erk was detected by immunoblotting (IB).