ESR1 and breast cancer: According to the intrinsic classification proposed by Perou and Sørlie in 2000 [22], (BC) can be divided into four molecular subtypes: Luminal A and Luminal B (both characterized by estrogen receptor (ER) expression), basal-like (lacking ER, progesterone receptor (PR), and epidermal growth factor receptor 2 (HER2) expression—commonly referred to as triple-negative breast cancer (TNBC)), and HER2-enriched (lacking ER but overexpressing HER2).