Considering the role of DLL1 cells in the regulation of BCSCs in ER+ tumors [17] and recent findings showing DLL1+ tumor cells confer chemoresistance to conventional therapy [20], our findings strengthen the interest in evaluating the anti-cancer potential of Dl1.72 mAb in aggressive metastatic models of ER+ BC tumors to further evaluate its therapeutic potential, both as monotherapy or in combination with standard endocrine therapy. This evidence concerns the gene ESR1 and cancer.