ESR1 and breast cancer: Breast cancer is a life‐threatening disease with the highest incidence and mortality in women with cancers.1 In 2012, there were more than 1.7 million newly diagnosed cases and 521 900 deaths worldwide.2 Triple negative breast cancer (TNBC), accounting for 15–20% of breast cancer, is characterized by lacking the overexpression of estrogen receptors, progesterone receptors, and human epidermal growth factor receptor 2 (HER2).3, 4 With the highest rate of metastasis and frequent recurrence,5 TNBC is the most difficult subgroup to target and cure compared to other forms of breast cancer.