In contrast, during several chronic infections and cancer, due to persistent antigen stimulation, antigen-specific T cells become dysfunctional or even exhausted, which is characterized by a decreased effector function, the sustained expression of various inhibitory receptors, such as PD-1, TIM-3, and cytotoxic T lymphocyte antigen 4 (CTLA-4), and a loss of memory ability [9,10,11]. The gene discussed is PDCD1; the disease is cancer.