It received accelerated approval from the US Food and Drug Administration (FDA), in combination with the aromatase inhibitor (AI) letrozole, for the treatment of postmenopausal women with estrogen receptor–positive (ER+), HER2-negative (HER2–) metastatic breast cancer (MBC) as initial endocrine-based therapy for metastatic disease (Pfizer Inc., 2015). This evidence concerns the gene ESR1 and metastatic neoplasm.