PDCD1 and cancer: Immune checkpoint-based immunotherapies (ICIs) such as the antibodies that target immunomodulatory receptor, cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), or to its ligand, programmed death ligand 1 (PD-L1), have revolutionised the treatment of a variety of cancer types including melanoma [7,8,9,10,11].