15 years ago, a woman in her thirties was diagnosed with metastatic kidney cancer. The oncologist, José Pablo Maroto, decided to treat her with the drug temsirolimus, which worked much better than expected. The patient beat the cancer. Nine years later, bone metastasis was detected, but temsirolimus was once again effective. Now, we know why temsirolimus proved so effective in this case and in that of two other patients. This finding will allow clinicians to identify other kidney cancer patients for whom temsirolimus will likely be the ideal treatment. At the moment, drugs like temsirolimus—so-called mTOR inhibitors—are only used when...