ERBB2 and breast cancer: As a heterogenous disease, breast cancer classification according to the molecular type of tumor is based on the immunohistochemical expression of estrogen and progesterone receptors together with the ki67 index [(luminal A (LA) and luminal B (LB)], the expression of human epidermal growth factor receptor 2 (HER2), or the absence of these receptors, known as triple-negative (TN).