Our recent study also showed that CCL2 produced by MMP-11-overexpressing macrophages activates the MAPK pathway through its receptor CCR2 in breast cancer cells, thereby enhancing the migration of HER2-positive breast cancer cells by increasing the expression of MMP-9, illustrating the regulation of MMP-9 by the MAPK pathway (57). The gene discussed is MMP9; the disease is breast cancer.