CCL2 and breast cancer: Overexpression of CCL2 in lung epithelial cells dramatically increased the number of metastatic foci without affecting the growth of the primary tumor and MDSC infiltration in the lung but not in the primary tumor, leading to the conclusion that high expression of CCL2 in the lung was associated with MDSC recruitment and contributed to the establishment of a microenvironment that promoted lung metastasis of BC cells [104].