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Embarking on checkpoint-inhibitor immunotherapy for cancer is a bit like taking a single pull on the lever of a slot machine. For a relatively small risk — such drugs are generally safer than other types used to treat cancer — recipients can win a massive reward: years of disease-free survival. “My longest responder is from 2001, and she continues to do well long term,” says Antoni Ribas, an oncologist at the University of California, Los Angeles. However, only a small proportion of people who are eligible for treatment with the drugs reap that reward — less than 40% for melanoma, for example, the type of cancer for which treatment has been most successful.