Treatment for TNBC [tumors that are estrogen receptor (ER)-negative, progesterone receptor (PR)-negative and human epidermal growth factor receptor 2 (HER2) non-amplified] continues to be a challenge due to the fact that it is, by definition, insensitive to the hormonal therapies and trastuzumab that have been developed to treat other types of breast cancer. Here, ESR1 is linked to breast cancer.