PDCD1 and cancer: Immune checkpoint inhibitors (ICIs) targeting programmed cell death protein 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) have been at the forefront of all anti-cancer immunotherapies and are currently approved for the treatment of various cancers, including those of melanoma, lung, bladder, gastric, cervix, head, neck, Merkel cell, hepatocellular, cutaneous squamous cell cancers, classic Hodgkin’s lymphoma, and B-cell lymphoma [7].