PDCD1 and neoplasm: Immune checkpoint inhibitors (ICIs) are novel modulators of immune homeostasis, targeting inhibitory receptors [e.g., programmed cell death protein 1 (PD-1), cytotoxic T lymphocyte antigen 4 (CTLA-4)] and ligands (PD-L1) expressed on T lymphocytes, antigen-presenting cells and tumor cells and aiming to stimulate immune cells to elicit an anti-tumor response.