They express inhibitory receptors, such as PDCD1, TIM-3, TIGIT, LAG-3 and cytotoxic T-lymphocyte-associated protein 4 (CTLA4) [47,48,49], which induce T-cell exhaustion and prevent T-cell activation; however, cancer cells utilize these receptors to evade T-cell attacks [50,51,52]. Here, PDCD1 is linked to cancer.