Colon cancer is often driven by underlying defects in DNA repair during cell division called “mismatch repair deficiency”. This is a test that is easily looked at on tumor tissue and found in about 15-20% of colon cancers. These defects in DNA repair lead to increased rates of DNA mutation in cancer cells that significantly alter a cancer cell compared to a person’s normal cells. Tumors with this defect are often more sensitive to chemotherapy that works by making DNA repair even more defective to the point where a cancer cell can no longer function and dies. New research presented recently at ASCO looked into whether your immune system can help identify and destroy these “defective DNA cancer cells” through immunotherapy. Immunotherapy works to supercharge your immune system to identify and kill cancer cells just like it would with an infection. In this trial, they found that over 60% of patients with “mismatch repair deficiency” in colon cancer cells that were otherwise resistant to chemotherapy had a shrinkage in cancer cells with immunotherapy alone. This study needs to be confirmed in further trials before being approved by the FDA, but shows how encouraging the field of immunotherapy is going to be in many kinds of cancer.