Based on molecular and histological evidence, BC could be categorized into three groups: (1) BC expressing hormone receptor (estrogen receptor (ER+) or progesterone receptor (PR+)) commonly noted as luminal tumors and are responsive to endocrine therapy, (2) BC expressing human epidermal receptor 2 (HER2+) which is characterized by the overexpression of HER2 oncogene and is treated with trastuzumab, (3) Triple-negative breast cancer (TNBC) (ER−, PR−, HER2−) subtype, which is associated with high mortality rates and is not responsive to some drug treatment approaches [4,5]. The gene discussed is ESR1; the disease is breast cancer.