ESR1 and breast cancer: Specifically, this includes around 30% of patients with human epidermal growth factor receptor 2 (HER2)-positive breast cancer (HER2BC, an overexpression of HER2), 30–50% of those with triple-negative breast cancer (TNBC, lacking expression of estrogen receptor (ER), progesterone receptor (PR), and HER2), and 15% of patients with hormone receptors (HR)-positive breast cancer (HRBC, an overexpression of ER or PR) [5, 6].