ERBB2 and breast cancer: Background: Breast cancer is the most commonly diagnosed neoplasm in women and is classified into different molecular subtypes based on the expression characteristics of estrogen and progesterone receptors (ERs and PRs) and human epidermal growth factor 2 (HER2, ERBB2): Luminal A, Luminal B, HER2(+), and triple-negative breast cancer (TNBC).