PDCD1 and cancer: Immune-checkpoint inhibitor (ICI) therapies have shown unprecedented durable responses in patients with advanced-stage cancers, including the success of anti-programmed cell death protein 1 (PD-1), anti-programmed death-ligand 1 (PD-L1) and anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), but the response rate is only 10%-40% [1, 2].