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Experiments involving just a few patients have hinted at it, and research on one type of tumor at a time has supported it, but a large study has now delivered the strongest evidence yet about how to identify cancer patients who are likely to benefit from a particular form of immunotherapy: count.

Specifically, count how many mutations their tumor cells have. The higher this “tumor mutational burden,” concludes a study published on Monday in Nature Genetics, the likelier a patient is to go into remission, and possibly be cured, by checkpoint inhibitor drugs such as Bristol-Myers Squibb’s nivolumab (Opdivo) and Merck’s pembrolizumab (Keytruda).


In the study’s main surprise, what counts as a high mutational burden differs markedly from one type of cancer to another: As few as six mutated genes per million DNA bases means a breast cancer is likely to respond to immunotherapy, but a colorectal cancer needs 52 to be vulnerable.

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