Hormone receptor (HR)-positive [estrogen (ER) and/or progesterone (PgR) receptor-positive] BC comprises the most common type of BC, whereas amplification or overexpression of the human epidermal growth factor receptor 2 (HER2) oncogene is present in approximately 20% of invasive BC, half of which also express HR [1]. This evidence concerns the gene ESR1 and breast cancer.