ERBB2 and breast neoplasm: Breast tumors are thus divided into four main subtypes based on immunohistochemical properties and hormone receptor (HR) status [3]: (1) HR+/HER2+, tumors which are sensitive to either estrogen or progesterone and present HER2 receptors, (2) HR+\HER2−, tumors which are sensitive to either estrogen or progesterone but lack HER2 receptors, (3) HR−\HER2+, tumors which express HER2 receptors, but lack hormone receptors and (4) HR−\HER2−, also known as triple-negative breast cancer (TNBC).