ERBB2 and breast cancer: By binding to and preventing activation of an intracellular tyrosine kinase domain on the HER2 receptor, trastuzumab stimulates cell-cycle arrest and apoptosis in HER2-positive cells.54 Large, well-designed trials have demonstrated a survival benefit with trastuzumab administration in both early and advanced HER2-positive breast cancer, and it is now standard of care in HER2-positive disease.55,56