In many cancers, the tumor microenvironment shows dysregulated expression of immune-checkpoint molecules, including Programmed cell death protein 1 (PD-1), Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4), T-cell Immunoglobulin and Mucin-domain containing-3 (TIM-3), and Lymphocyte Activation Gene-3 (LAG-3) (72, 73). Here, PDCD1 is linked to cancer.