PDCD1 and cancer: Recent success in using antibodies against various immune checkpoints such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed death 1 (PD-1), and programmed death ligand 1 (PD-L1) for cancer immunotherapy has brought this approach being implemented as a new treatment modality for CRC, especially in terms of targeting the microsatellite instability-high (MSI-H) phenotype (Le et al., 2015; Overman and McDermott 2017; Overman et al., 2018; Le et al., 2020).