These proteins, including programmed cell death-1 (PD-1), cytotoxic T-lymphocyte antigen-4 (CTLA-4), and T-cell immunoglobulin and mucin domain 3 (TIM3), represent the immune regulatory function of co-inhibitory receptors associated with T-cell dysfunction and exhaustion in many cancers [6–8]. The gene discussed is PDCD1; the disease is cancer.