In women that are ≥70 years old with clinical stage I (T1N0) estrogen-receptor positive (ER+) breast cancer on endocrine therapy, the Cancer and Leukemia Group B (CALGB) 9343 clinical trial data initially published in 2004 increasingly supports that lumpectomy alone compared to lumpectomy and breast radiotherapy results in equivalent survival and acceptable cancer control (5, 6). Here, ESR1 is linked to breast cancer.