PDCD1 and metastatic colorectal cancer: Currently, ipilimumab combination with nivolumab, a programmed cell death protein 1 (PD-1) inhibitor, has been approved to treat various solid tumors, including melanoma [6], renal cell carcinoma [7], non-small cell lung cancer (NSCLC) [8, 9], metastatic colorectal cancer with microsatellite instability high or mismatch repair deficiency [10], hepatocellular carcinoma (HCC) [11], malignant pleural mesothelioma [12], and esophageal cancer [13].