PDCD1 and neoplasm: Malignant cells or persistent infections can dysregulate the expression of ICPs, such as CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), PD-1 (programmed cell death protein 1), PD-L1 (programmed death-ligand 1), TIM-3 (T-cell immunoglobulin mucin protein 3), and LAG-3 (lymphocyte-activation gene 3) on the surface of immune cells to evade or subvert the immune response, leading to insufficiency or failure in anti-tumor or pathogen immune attacks [22, 26–28].