M2 macrophages can secrete chitinase 3-like protein 1 (CHI3L1) to interact with interleukin-13 receptor α2 (IL-13Rα2) chain on the plasma membranes of cancer cells to activate the mitogen-activated protein kinase (MAPK) signaling pathway, by which the expression of MMP genes are upregulated, leading to the promotion of the metastasis of breast cancer cells [150]. The gene discussed is CHI3L1; the disease is cancer.