PDCD1 and cancer: Specifically, immune checkpoint inhibitors (ICIs) that target inhibitory immune-checkpoint receptors such as cytotoxic T lymphocyte antigen 4 (CTLA-4), programmed cell death 1 (PD-1), and programmed cell death 1 ligand 1 (PD-L1) have been approved by the Food and Drug Administration (FDA) for treating over nineteen cancer types and two tissue-agnostic conditions [4,5].