To show for the first time that the correlation between ESR1 expression and EERES is important for endocrine therapy responsiveness, we analyzed the survival of ER-positive/HER2-negative breast cancer patients (n = 436) using tumor sample data from the TCGA Pan-Cancer Atlas and developed an algorithm to select patients with better survival by selecting patients based on both their ESR1 expression and EERES. Here, ESR1 is linked to cancer.