In addition to these variations of histological types of breast cancer, there are hormonal variations in breast cancer such as estrogen receptor-positive (ER+) or -negative (ER−), progesterone receptor-positive (PR+) or -negative (PR−), and human epidermal growth factor receptor 2-positive (HER2+) or -negative (HER2−) cancers. Here, ESR1 is linked to cancer.