ESR1 and breast cancer: Importantly, the results indicated that MCM3 was not only a prognostic marker in adjuvant tamoxifen-treated early-stage breast cancer patients, but also a predictive marker of tamoxifen benefit, as we found a correlation between MCM3 levels and outcome in the tamoxifen adjuvant treatment arm, but not in the no-adjuvant-treatment arm of ER+ breast cancer patients.