ESR1 and breast cancer: Breast cancer is a heterogeneous disease with different etiologies, clinicopathological characteristics, and responses to treatment.(1)Breast cancer is currently divided into five subtypes, derived from the Perou et al. classification (2000).(2)Based on immunohistochemistry, tumors expressing the estrogen receptor (ER) and/or progesterone receptor (PR) are luminal-like breast cancers.