ESR1 and breast neoplasm: Expression of these receptors influences treatment decisions as the presence of the estrogen receptor (ER), expressed in ~80% of breast tumors (18), determines a tumor's response to endocrine therapy while expression of HER2 (19) means that the cancer can be treated with monoclonal antibodies that specifically target this receptor, such as trastuzumab (herceptin) (20, 21).