ESR1 and breast cancer: In BC, Ki-67 is traditionally used in combination with hormone receptor (HR) status and HER2/neu status to provide an IHC4 score used alongside clinical prognostication tools to identify patients who require cytotoxic chemotherapy in addition to endocrine therapy [12], and its expression is becoming increasingly essential to refrain from over- or undertreatment in respect to the other traditional factors such as TNM (tumor, node, and metastasis) staging, ER/PR+, and HER2/neu- of early BC [13].