ERBB2 and breast cancer: Estrogen/progesterone receptor-positive (ER/PR+) human epidermal growth factor receptor 2 negative (HER2/Neu-) MCF-7, metastatic MDA-MB-231 (ER/PR-; HER2/Neu-), and BT-20 (ER/PR-; HER2/Neu-) tumorigenic human epithelial breast cancer cell lines were used for this study to eliminate any potential hormone receptor involvement.