PDCD1 and cancer: Several ICBs, including CTLA4 antibody (anti-CTLA4) and the PD-1/PD-L1 inhibitors (anti-PD-1/anti-PD-L1), have been approved to treat a variety of cancers, such as melanoma, microsatellite instability-high/mismatch repair-deficient metastatic colorectal cancer (mCRC), NSCLC and hepatocellular carcinoma (HCC) etc. However, not all cancer patients respond to ICBs therapy.