ESR1 and neoplasm: For example, in the case of ER+ tumor estrogen-receptor modulators, such as tamoxifen and letrozole are administered4, trastuzumab (Herceptin) is a humanized monoclonal antibody developed to target and inhibit the function of HER2 and a dual anti-HER2 regimen, pertuzumab in combination with trastuzumab and docetaxel are administered, however, the incidences of adverse events and resistance to these drugs are not uncommon5.