Myeloid-derived suppressor cells (MDSCs) are a subset of immature myeloid cells that inhibit anti-tumor immunity and contribute to poor cancer outcomes. In this study, the authors used multi-color flow cytometry to detect changes in MDSCs in patients with cancer and tumor-bearing mice. Then the authors studied changes in MDSCs ratio and mouse tumors after administration of hypoxia-inducible factor 1α (HIF-1α) inhibitor. The results showed that the ratio of MDSCs, specifically polymorphonuclear MDSCs (PMN-MDSCs), was higher in patients with cancer, and both PMN-MDSCs and monocytic MDSCs (M-MDSCs) ratio were higher in tumor-bearing mice. When provided with the HIF-1α inhibitor LW-6, the ratio of MDSCs decreased in tumor-bearing mice, particularly PMN-MDSCs, and the volume of liver metastases also decreased. The authors' findings suggest that reducing MDSCs by inhibiting hypoxia-inducible factor 1α may slow tumor progression.