Endocrine therapies targeting ER, such as selective ER modulators (e.g., tamoxifen), selective ER degraders (e.g., fulvestrant) and aromatase inhibitors (e.g., letrozole and anastrozole), have been effective in reducing recurrence for up to 5 years in early ER+ breast cancer (Bray, et al., 2018; DeSantis, et al., 2017; Williams and Harris, 2014). This evidence concerns the gene ESR1 and breast cancer.