PDCD1 and cancer: ICIs are monoclonal antibodies that stimulate the anti-tumour immune response by inhibiting checkpoint proteins such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death-1 (PD-1), its primary ligand PD-L1 and other expressed on T cells, antigen-presenting cells (APCs) and cancer cells [5].