PDCD1 and neoplasm: Gene profiling and phenotypical studies in human and mice have shown that exhausted tumor-infiltrating T lymphocytes typically express high level of inhibitory receptors including cytotoxic T lymphocyte antigen-4 (CTLA-4, CD152), programmed death-1 (PD-1, CD279), lymphocyte activation gene 3 protein (LAG3), T cell immunoglobulin domain and mucin domain-containing protein 3 (TIM-3, HAVCR2), 2B4 (CD244), CD160, TIGIT, and many other inhibitory molecules (Fourcade et al., 2012; Qin et al., 2019).