ESR1 and breast carcinoma: Based on the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2), breast cancers are classified into three distinct molecular subtypes as follows: hormone receptor (HR) positive and HER2 negative (HR+/HER2−), HER2+, and triple-negative breast cancer (TNBC) (3).