We uncovered a significant reduction in estradiol secretion by ER+ breast cancer cells (Fig. 4A), ER− breast cancer cells (Supplementary Fig. S3A), and normal breast cells (Supplementary Fig. S3B) upon ionizing radiation, and thus used real-time quantitative PCR assay to determine the mRNA expression for 17β-HSD, CYP17A, and CYP19A enzymes of the estradiol synthetic pathway. Here, ESR1 is linked to breast cancer.