PDCD1 and neoplasm: ICIs function by targeting the key regulatory pathways of cytotoxic T-cells, such as CTLA-4, the programmed cell death protein-1 (PD-1)/programmed death ligand-1 (PD-L1) axis, and more recently, lymphocyte activation gene 3 (LAG3), effectively releasing the “brakes” on T-cells and enhancing their ability to eliminate tumor cells [7,8].