ESR1 and breast cancer: Triple‐negative breast cancer (TNBC) is defined as human epidermal growth factor receptor 2 (HER2)‐negative, progesterone receptor (PR)‐negative, and estrogen receptor (ER)‐negative breast cancer, which comprises 15%–20% of all cases and shows a higher recurrence rate as well as worse prognosis than other subtypes with a median survival time of only 1–1.5 years after the diagnosis of metastatic disease.2, 3, 4