TNFRSF9 and cancer: However, the aim of this review was to present the latest findings on how the CD137 receptor is alone able to identify a population capable of recognizing and eliminating cancer cells, thus discussing the potential use of this population as a biomarker to evaluate the patients’ capability to respond to immune-based therapies as well as the potential benefit of introducing CD137+ T-cells as protagonists of novel strategies for the treatment of oncologic patients.