Here, our study of the in vivo synthetic MN model addresses the controversy about Gr1+CD11b+Ly6G+ neutrophils in breast cancer by revealing the pro-tumor roles of neutrophils in regulating lung-tropic metastatic dissemination of breast cancer cells and supporting metastatic outgrowth as well as their antitumor roles in boosting antitumor immunity and driving metastatic dormancy in some conditions. The gene discussed is ITGAM; the disease is breast carcinoma.