CCL2 and cancer: Of note, MMP9 identified in our work has also been described as a culprit of organ‐specific metastases and is mainly expressed by infiltrated macrophages in pre‐metastatic organ.9 The previous study further supports our observation of significantly reduced MMP9 expression as well as cancer cell extravasation in the pre‐metastatic lung of CCR2‐DTR mice, in which recruitment of inflammatory monocytes via CCL2‐CCR2 axis was blocked, indicating primary tumor‐induced recruited inflammatory monocytes and subsequent macrophages were responsible for MMP9 expression in the pre‐metastatic lung.