ERBB2 and breast cancer: A recent study showed that the documented immune checkpoint PD-L1/PD-1 is critically involved in trastuzumab resistance in breast cancer.34,35 Consistent with previous reports, we observed that IFN-γ significantly increased the level of PD-L1 in breast cancer cells (Fig. 5a).36 Blockade of PD-L1 increased the cytotoxicity of NK cells against trastuzumab-treated HER2-overexpressing breast cancer cells (Fig. 5b).