ESR1 and breast carcinoma: Breast cancer can be classified into four distinct molecular subtypes based on immunohistochemical examination of samples obtained through biopsy procedures: HER2−/HR+ (estrogen receptor (ER)/progesterone receptor (PR)-positive, human epidermal growth factor receptor 2 (HER2)-negative), HER2+/HR+ (ER/PR-positive, HER2-positive), HER2-enriched (ER/PR-negative, HER2-positive), and TNBC (triple-negative breast cancer, lacking all three receptors).