Cell viability in MCF-7 and SK-Br-3 cells. Mean (± SEM) cell viability determined using a tetrazolium conversion assay. Cells were first exposed to trastuzumab ranging from 1 μg/ml to 500 μg/ml. The result was a reduction in cell viability in SK-Br-3 cells which was not seen in MCF-7 cells. Cells were then exposed to a saturating concentration of trastuzumab combined with either EGF (400 ηg/ml) or HRG-β1 (200 ηg/ml). These endogenous biological molecules significantly abrogated cell viability observed for trastuzumab (Table1).