ESR1 and breast cancer: BC is characterized by its significant histological heterogeneity, with the majority of cases classified as “no special type” (NST) and further categorized into Luminal A, Luminal B, HER2-enriched, and triple-negative BC (TNBC) based on the expression of estrogen receptor (ER), progesterone receptor (PR), Ki-67, and HER2 receptor [2].