Inhibitory receptors, such as programmed cell death 1 (PD-1), lymphocyte activation gene-3 (LAG-3), T-cell immunoglobulin-3 (TIM-3), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), are highly expressed in tumor-infiltrated CD8+ T cells, which results in CD8+ T-cell exhaustion [33, 34]. Here, PDCD1 is linked to neoplasm.