ESR1 and breast cancer: Specifically, the presence on the surface of neoplastic cells of the estrogen receptor (ER) and the progesterone receptor (PR), together with the expression and amplification of the human epidermal growth factor type 2 (HER2) receptor, allows for the classification of BC into three main clinical subtypes, namely, ER/PR positive, HER2 positive, or triple negative (TNBC) [3].