ESR1 and breast cancer: Post-menopausal hormone-positive breast cancer patients. Histological diagnosis of invasive carcinoma of the breast. X-ray evidence (mammography and / or ultrasound) strongly suggestive for the presence of invasive breast cancer (BIRADS 4c or BIRADS 5) of greater than 15mm diameter.Positivity for the estrogen receptor and / or to the progestin defined as the expression of one or both hormone receptors in ≥10% of tumor cells, negativity for HER2.