PDCD1 and metastatic colorectal cancer: DNA mismatch repair deficient (dMMR) and microsatellite instable (MSI) metastatic colorectal cancer (mCRC) can be successfully treated with FDA- and EMA-approved immune checkpoint inhibitors (ICI) pembrolizumab and nivolumab (as single agents targeting the anti-programmed cell death protein-1 (PD-1)) or combinations of a PD-1 inhibitor with ipilimumab, a cytotoxic T-lymphocyte-associated protein 4 (CTLA-4)-targeting antibody.