Breast cancers are mainly treated with surgery, radiotherapy, chemotherapy, SERMs (tamoxifen), estrogen receptor down-regulators (fulvestrant), aromatase inhibitors (anastrozole, exemestane, letrozole), several drugs (ado-trastuzumab emtansine, lapatinib, palbociclib, pertuzumab, and trastuzumab), antimetabolites (methotrexate), DNA-interactive agents (cisplatin, doxorubicin), antitubulin agents (taxanes), etc. [11,12]. The gene discussed is ESR1; the disease is breast cancer.