ESR1 and breast cancer: Basal-like breast cancers are distinguished by a high expression of CK5, CK14, caveolin-1, caix, p63, and EGFR (epidermal growth factor receptor gene)/HER1, which focus mainly on the basal/myoepithelial cell components of the mammary gland, along with reduced expression of PR, ER, and HER2 [10].