PDCD1 and cancer: Interestingly, targeting multiple immune checkpoints has been demonstrated to be a more effective approach to activate anti-tumor immune responses than single immune checkpoint-specific monotherapy [4], which makes the combinations, such as CTLA-4 (cytotoxic T lymphocyte-associated protein-4) blocker plus PD-1/PD-L1 (programmed cell death protein-1/programmed death-ligand 1) blocker, a very promising anti-cancer therapeutic strategy, including against breast cancer [4,9].