ERBB2 and breast cancer: Human Epidermal Growth Factor Receptor-2 (HER2) positive status, defined as HER2 protein overexpression (immunohistochemistry 3+) and/or HER2 gene amplification, is observed in about 15–25% of breast carcinomas overall [4], and less frequently in ≤ 1 cm breast cancer (10%) [5,6].