ERBB2 and breast cancer: Considering that oleuropein aglycone-promoted cell growth inhibition and apoptosis was significantly higher in HER2-overexpressing than in HER2-negative breast cancer cells, which was concomitant with a significant depletion of the HER2 oncoprotein, we hypothesized that oleuropein aglycone may represent a novel biological anti-HER2 agent capable to reverse auto-acquired resistance to trastuzumab-based treatments.