ESR1 and breast carcinoma: Breast cancer is typically divided into four molecular subtypes: Luminal A, Luminal B, human epidermal growth factor receptor 2 (HER2)-positive, and triple negative [2]. Approximately 80% of cases fall within the luminal A or B categories, both characterized by the expression of the estrogen receptor (ER) [3], which drives their growth and response to hormone therapy.