CD274 and breast cancer: To further sustain the higher expression of PD-L1 in the CSC compartments, we examined the tumor-forming ability of the murine SN25A mammary tumor cells [30] sorted according to PD-L1Highversus PD-L1Low expression (Table 1) within the gate of SCA-1-positive cells, a murine biomarker for CSCs [31] (Supplementary Fig. S6), and injected them at two different dilutions (103 and 102) into the mammary fat pad of BALB/c mice.