MDSCs express the death receptor Fas. Leukocytes from the blood of tumor-free BALB/c mice, BALB/c mice with 4T1 or TS/A mammary tumors, C57BL/6 mice with AT3 mammary tumors, and TIMDSCs from BALB/c mice with 4T1 tumors were labeled for Gr1, CD11b, and Fas, and the Gr1⁺CD11b⁺ cells were gated and analyzed by flow cytometry for expression of Fas. (A) Flow cytometry of cells from individual mice showing Fas expression on the gated CD11b⁺Gr1⁺ cells. (B) Percentage +SD of Fas⁺CD11b⁺Gr1⁺ cells from 2 experiments (n = 6 mice/group).