While CTLA-4 binds to CD28 ligands (CD80/CD86) to hinder T-cell priming, PD-1 attaches to its ligands (PD-L1/PD-L2) on tumor cells to induce T-cell death and inhibit the effector functions of cytotoxic T cells (Tc cells) [28,29,30,31]. Here, PDCD1 is linked to neoplasm.