BC is sub-divided into four subtype categories according to the hormone receptor status: Luminal A (ER + , progesterone receptor (PR) + , and human epidermal receptor 2 (HER2)-); Luminal B (ER + and/or PR + , HER2 +); HER2 overexpressing (ER-, PR- and HER2 +); and triple negative (ER-, PR- and HER2-) [27]. Here, ESR1 is linked to breast cancer.