Different breast cancer subtypes, as well as cell lines, show different levels of radiosensitivity [24, 25], for example, significantly improved overall survival after radiation was seen in patients with Estrogen receptor (ER)+ and Human Epithelial Growth Factor Receptor-2 (HER2)– tumors, but not in patients with ER– and HER2+ tumors. Here, ESR1 is linked to breast cancer.