PDCD1 and breast neoplasm: To examine acquired resistance to PD-L1 inhibition, the PD-1 pathway inhibitor-sensitive murine breast tumor EMT6 cell line (Clift et al, 2019; Lan et al, 2018; Schofield et al, 2021) was implanted orthotopically in BALB/c mice and treated with αPD-L1 (Clone 80) or IgG control antibody (Fig. 1A, schematic shown).