PDCD1 and cancer: CD8+ T-cells are essential for immune-mediated tumor elimination, but cancer cells can inhibit these cells’ cytotoxic function by activating immune checkpoints, including PD-1, cytotoxic T-lymphocyte-associated protein 4 (CTLA4), T-cell immunoglobulin and mucin-domain containing-3 (TIM-3), and lymphocyte-activation gene 3 (LAG-3) [10, 11].