PD-1 is one of the major inhibitory receptors that regulates T-cell exhaustion and the T cells with high PD-1 expression fail to eradicate cancer [48], thus making PD-1 an attractive target in cancer immunotherapy using anti-PD-1 monoclonal antibody, although the reported objective response rates were around 20% to 30% in different single-arm trials [4]. Here, PDCD1 is linked to cancer.