ESR1 and breast cancer: This subtype is a clinical challenge, as it does not respond to standard endocrine therapies, such as tamoxifen (an anti-estrogen agent against the estrogen receptor) and trastuzumab (a monoclonal antibody against HER2), and is, therefore, associated with worse prognosis and survival, increased metastases, and higher risk of recurrence compared to the other BC subtypes [15].