The Keynote 117 phase III trial demonstrated the superiority of pembrolizumab (anti-PD1 monoclonal antibody) versus chemotherapy +/- targeted therapy in first-line treatment of dMMR/MSI metastatic colorectal cancer (mCRC). However, primary resistance to pembrolizumab was observed in approximately 20-30% of patients treated in the Keynote 177 study. Therefore, the identification of biomarkers predictive of resistance to immunotherapy for dMMR/MSI mCRC is necessary to better select patients who benefit the most from immunotherapy, and those for whom other therapeutic approaches should be favored.