ESR1 and breast cancer: In two recent studies, we report that peripheral blood monocytic myeloid-derived suppressor cells (Mo-MDSCs; CD14+HLA-DRlow/− cells) are enriched in MBC patients compared to healthy donors and that high levels of Mo-MDSCs correlate with ER-negativity, metastatic disease at the initial breast cancer diagnosis (de novo MBC) and with worse outcome [22,23].