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Cancer cells create multiple mutated proteins (i.e., neoantigens). Theoretically, each of these neoantigens should be recognized as foreign and therefore be attacked by the immune system. But that typically doesn't happen.
In a 49-year-old woman with estrogen receptor–positive, HER2-negative metastatic breast cancer that was refractory to chemotherapy, investigators performed whole-exome sequencing of the tumor DNA, as well as determination of all its messenger RNAs to identify several neoantigens. With cell sorting, they identified tumor-infiltrating T-lymphocytes in the patient's blood that were targeting those neoantigens. Then, they sequenced the messenger RNAs inside individual tumor-infiltrating lymphocytes to confirm that those cells …