ERBB2 and breast neoplasm: Clinically, breast tumors are classified into hormone receptor positive (HR+) tumors expressing the estrogen (ER) and/or progesterone (PR) receptors, human epidermal receptor 2 (HER2)-enriched tumors with HER2 overexpression in the absence of HR expression, and triple negative breast cancer (TNBC) lacking expression of all three receptors.