ESR1 and breast cancer: For example, breast cancer with estrogen receptor (ER) and progesterone receptor (PR) expression has been associated with sensitivity to endocrine therapy [7, 8], whereas human epidermal growth factor-2 (HER2)-overexpressed or HER-2-amplified breast cancer is resistant to endocrine therapy [9], but is more sensitive to trastuzumab [10, 11].