ESR1 and breast cancer: Triple‐negative breast cancer (TNBC), a type of breast cancer devoid of expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor‐2 (HER2),[1] has a highly aggressive clinical course with earlier age of onset, greater metastatic potential and poorer clinical outcomes.[2] Due to its special molecular phenotype, TNBC is impervious to endocrine therapy or molecular targeted therapy.