Some studies have indicated that tumor-secreted CCL20 activates granulocyte–monocyte progenitor cell differentiation through its receptor CCR6, leading to the expansion of PMN-MDSCs, which activates the CXCR2/NOTCH1/HEY1 signaling pathway to increase ALDH+ breast cancer tumor stem cells [49]. Here, CXCR2 is linked to breast cancer.