ERBB2 and breast cancer: Depending on expression of estrogen receptor alpha (ERα), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), breast cancer (BC) is classified as ERα-positive (ERα+/PR+/HER2±), EGFR2-overexpressing (ERα-/PR-/HER2+) and triple negative (ERα-/PR-/HER2-), with ≈70% of all BC cases categorized as ERα-positive [1].