ESR1 and breast cancer: Breast cancer is one of the most prevalent malignancies and a leading cause of cancer-related mortality worldwide, with approximately 2.3 million new cases diagnosed, according to the World Health Organization (WHO) [1,2,3], and is classified into four molecular subtypes based on estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and the proliferation marker Ki-67.