Among 1405 women included in the intention-to-treat analysis, the median (IQR) age was 60 (52-68) years, the median (IQR) tumor size was 1.1 (0.8-1.5) cm, and 1234 patients (87.8%) had estrogen receptor (ER)–positive ERBB2 (formerly HER2 or HER2/neu), nonoverexpressing BC. This evidence concerns the gene ESR1 and breast cancer.