ESR1 and cancer: Recently-developed signatures aiming to predict intrinsic radiosensitivity such as the radiosensitivity signature (RSS) developed by Speers et al. at the University of Michigan [11] and the radiosensitivity index (RSI) developed by Torres-Roca et al. at Moffitt Cancer Center [37] showed disparate predictive and prognostic utility in ER + vs ER- tumors in an independent cohort [9].