In the present study, we found that after BM of BC cells, Arg1+ microglia were significantly aggregated at the site of tumor metastasis (Figure 1F), and the levels of cytokine IL6, chemokine CCL2, were significantly increased (Figures 1D,E), suggesting an immunosuppressive microenvironment following BC-BM. The gene discussed is CCL2; the disease is breast cancer.