PDCD1 and metastatic colorectal cancer: ICB, including programmed cell death protein-1 (PD-1) inhibitors and anti-cytotoxic T lymphocyte antigen-4 (CTLA-4) antibodies, have significantly improved patients’ survival in DNA mismatch repair-deficiency (dMMR)/microsatellite instability-high (MSI-H) metastatic colorectal cancer (mCRC) (3–8).