In this study, we developed a multiplex bisulfite sequencing assay for detecting GR promoter methylation and used it to investigate the prognostic and predictive significance of GR methylation in a cohort of ER+ patients from a placebo-controlled trial of adjuvant tamoxifen in premenopausal women with early breast cancer (CCTG MA.12). This evidence concerns the gene NR3C1 and breast carcinoma.