PDCD1 and cancer: Regulatory T cells (Tregs) support cancer progression through their immunosuppressive mechanisms expressing co-inhibitory molecules like programmed cell death-1 (PD-1), cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), T cell immunoglobin-3 (TIM-3), and T-cell immunoglobin and ITIM domain (TIGIT), that suppress T-cell activation and allow tumor cells to grow uncontrollably.