ESR1 and triple-negative breast carcinoma: At the molecular level, breast cancer is a heterogeneous disease, divided into hormone/estrogen-receptor-positive (HR+/ER+), human epidermal growth factor receptor-2-positive (HER2+) and ER/PR/HER2 triple-negative breast cancer (TNBC) with corresponding treatment strategies according to molecular subtypes (Nagini, 2017).