Biomarkers may be “prognostic” and separate patients into favorable or unfavorable groups or be “predictive” and direct those expressing or not expressing the biomarker to different therapies. The figure shows that a current list of validated predictive cancer biomarkers is limited to molecular entities that define a disease and to proteins that are intimately involved in the mechanism of action of a targeted therapy. Kanakry et al suggest a role for plasma EBV-DNA levels as a prognostic biomarker for patients with Hodgkin lymphoma. Establishing a role of EBV in the continued pathogenesis of Hodgkin lymphoma and developing therapies against this process would create the potential for plasma EBV-DNA to be a predictive biomarker. APL, acute promyelocytic leukemia; CML, chronic myelocytic leukemia; EGFR, epidermal growth factor receptor; ER/PR, estrogen receptor/progesterone receptor; GIST, gastrointestinal stromal tumor.