During each patient’s laboratory work-up, oncology specialists rely on findings related to the tumor’s estrogen receptor (ER) and progesterone receptor status, as well as the amplification and overexpression of the human epidermal growth factor receptor 2 (HER2), to direct treatment planning.^2,3

Data suggest that greater than one-fourth of patients with estrogen receptor–positive metastatic breast cancer who are treated with an aromatase inhibitor (AI) will develop a mutation in the ESR1 gene, conferring resistance to the AI. Response after AI failure is poor, leading to a search for better therapeutic options.