PDCD1 and cancer: In cancer, they express high levels of inhibitory receptors, including programmed cell death 1 (PD-1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), T-cell immunoglobulin mucin-3, and lymphocyte-activation gene 3, as well as showing impaired production of effector cytokines, such as IL-2, tumor necrosis factor alpha (TNF-α), and interferon gamma (92).