ESR1 and breast cancer: In general, breast cancers are classified into five subtypes based on the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), which are luminal A, luminal B, HER2‐enriched, basal‐like, and normal‐like breast cancer subtypes.[1] Both luminal A and B subtypes are ER expression‐positive, which account for around two thirds of all breast cancer subtypes.