EGF and neoplasm: Because basal levels of PD-L1 are almost undetectable in SKBR3 cells, and are unchanged following EGF or IFNγ treatment (Figure S5A), we used mouse KPB6 tumor cells, which show a robust PD-L1 response to IFNγ (Figure S5B), as an alternative model to confirm the PD-L1 surface expression phenotype.