ESR1 and breast cancer: For example, targeted therapies for a type of triple negative breast cancers (TNBC; 15–20% of breast cancers) have not been developed due to the limitation of conventional subtyping of breast cancer in the clinic based on the expression of three main types of receptors, such as estrogen receptor (ER), progesterone receptor (PR) and the human epidermal growth factor receptor 2 (HER2, also known as ErbB2) [42].