PDCD1 and cancer: According to the targets of immune checkpoint molecules which act as negative regulators of T cells function in cancer immunological process, there are three main types of ICIs so far: cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1) (Qin et al., 2019; Kotanides et al., 2020).