ESR1 and breast carcinoma: The gene array-based breast cancer molecular subtypes have been approximated using a panel of five surrogate markers (estrogen receptor (ER), progesterone receptor (PgR), human epidermal growth factor receptor-2 (HER2), epidermal growth factor receptor (EGFR), and cytokeratin-5 (CK5)) as assessed by immunohistochemistry [11].