PDCD1 and metastatic neoplasm: Programmed cell death 1 (PD-1) and its ligand, PD-L1 blockade immunotherapy, have demonstrated significant anticancer activity in treatment-naïve and refractory microsatellite instability-high (MSI-H)/mismatch repair-deficienct (dMMR) metastatic colorectal cancers, which represent less than 5% of metastatic diseases [1,2].