NOS2 and breast cancer: Ouzounova et al. recently showed in a transplantable murine BC model that tumor-infiltrated M-MDSCs promote inducible nitric oxide synthase (iNOS)-mediated EMT and cancer stem cell properties in tumor cells at the invasion frontline; then, PMN-MDSCs in metastatic lungs induce mesenchymal–epithelial transition (MET) in CTCs and restore their original phenotype to foster settlement and proliferation [85].