ESR1 and breast cancer: According to the expression of human epidermal growth factor receptor 2 (HER2), estrogen receptor (ER), and progesterone receptor (PgR), breast cancer can be classified into four distinctive molecular subtypes recommended as solid reference to the precise treatment and prognosis estimation [2], of which HER2 over-expression is observed in approximately 20–25% of invasive breast cancer and is associated with an inclination of early recurrence and distant metastasis suggestive of an inferior prognosis [3].