Cancer drugs called immune checkpoint inhibitors boost the immune system’s ability to find and destroy tumor cells, but they don’t work for most patients. Researchers at the Broad Institute of MIT and Harvard, Massachusetts General Hospital, and Brigham and Women’s Hospital have identified a protein, called SETDB1, that helps cancer cells evade the immune system. When the team blocked this protein in mouse models of cancer, immune cells increased their cell-killing activity, and tumors shrunk when the scientists treated the animals with immune checkpoint inhibitors.