PDCD1 and breast cancer: On the one hand, induction of PD-L1 expression in tumor cells may increase the sensitivity of cancer cells to PD-1/PD-L1 immune checkpoint blockade, such as Poly-(ADP-ribose) polymerase (PARP) inhibitors in breast cancer and MET proto-oncogene, receptor tyrosine kinase (MET) inhibitors in HCC.