Tumor Immunology Myeloid-derived suppressor cells (MDSCs) are immune cells that mediate immune suppression and are correlated with progressing cancer. How these cells arise and whether they can be therapeutically targeted akin to exhausted T cells are areas of active investigation. A persistent challenge in studying MDSCs has been the identification of MDSC-specific cell-surface markers that can facilitate their isolation and characterization. Using single-cell RNA sequencing in a mouse model of breast cancer, Alshetaiwi et al. defined gene signatures that distinguish MDSCs from other myeloid and granulocytic cells. They identified the protein CD84 to be a robust cell-surface marker for identification of MDSCs in both human and murine breast cancer. Whether their findings can be extended to MDSCs in other cancer settings remains to be seen. Sci. Immunol. 5, eaay6017 (2020).