ESR1 and breast carcinoma: Based on differences in the expression of key receptors such as estrogen receptor (ER), progesterone receptor (PgR), and human epidermal growth factor receptor 2 (HER2), as well as the proliferation marker Ki67, breast cancer is typically classified into four major subtypes: luminal A, luminal B, HER2-positive (HER2+), and triple-negative breast cancer (TNBC) (6–10).