PDCD1 and neoplasm: In the last decade, programmed death protein-1/programmed death-ligand-1 (PD-1/PD-L1) and cytotoxic T lymphocyte antigen 4 (CTLA-4) have become two of the most important pathways targeted by ICI and are involved in the induction and maintenance of immunotolerance within the tumor microenvironment [14].