ESR1 and breast cancer: Molecularly, BC is classified as luminal A-like, luminal B-like (HER2-/HER2+), HER2 enriched, and triple-negative breast cancer (TNBC) (Figure 2), based on the presence or absence of distinct molecular: estrogen receptor (ER), progesterone receptor (PR), HER2 and the proliferation marker Ki67 which classification is commonly used in clinical practice (Gupta et al., 2021; Banthia et al., 2022).