BETHESDA, Md.—National Cancer Institute researchers have produced an immune-cell therapy that for the first time successfully targeted a genetic mutation involved in causing tens of thousands of gastrointestinal cancers.

The research, published in the New England Journal of Medicine on Wednesday, focused on only one patient whose metastatic colon cancer was completely reversed. But that patient’s unusual story may hold promise for many others, doctors said. The therapy targets a gene mutation estimated to drive more than 50,000...