Tumor vaccines can help the body fight cancer. These vaccines alert the patient’s immune system to proteins that are carrying cancer-typical alterations. Physicians and cancer researchers have now treated adult patients with advanced midline gliomas, difficult-to-treat brain tumors, with a peptide vaccine for the first time. The vaccine mimicked a mutational change in a histone protein typical of this type of cancer. The vaccine proved to be safe and induced the desired immune responses directed against the brain tumor.